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United States Patent |
6,183,668
|
Debe
,   et al.
|
February 6, 2001
|
Membrane electrode assemblies
Abstract
Membrane electrode assemblies are described that include an ion conductive
membrane a catalyst adjacent to the major surfaces of the ion conductive
membrane and a porous particle filled polymer membrane adjacent to the ion
conductive membrane. The catalyst can be disposed on the major surfaces of
the ion conductive membrane. Preferably, the catalyst is disposed in
nanostructures. The polymer film serving as the electrode backing layer
preferably is processed by heating the particle loaded porous film to a
temperature within about 20 degrees of the melting point of the polymer to
decrease the Gurley value and the electrical resistivity. The MEAs can be
produced in a continuous roll process. The MEAs can be used to produce
fuel cells, electrolyzers and electrochemical reactors.
Inventors:
|
Debe; Mark K. (Stillwater, MN);
Larson; James M. (Saint Paul, MN);
Balsimo; William V. (Afton, MN);
Steinbach; Andrew J. (Saint Paul, MN);
Ziegler; Raymond J. (Glenwood City, MN)
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Assignee:
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3M Innovative Properties Company (Saint Paul, MN)
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Appl. No.:
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208695 |
Filed:
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December 10, 1998 |
Current U.S. Class: |
252/510; 29/623.5; 204/296; 252/502; 252/519.3; 252/519.33 |
Intern'l Class: |
H01B 001/06 |
Field of Search: |
252/502,510,519.3,519.33
427/115
29/623.5
204/296,295,290.05,290.07,290.11,290.15
428/172-174,221,304.4,306.6,307.3,307.7,312.2,317.9,323
521/27
|
References Cited
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|
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| |
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| |
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| |
Other References
Acc. No. 88-144518/21 (Derwent Abstract) Apr. 18, 1988.
Acc. No. 88-311503/44 (Derwent Abstract) Sep. 26, 1988.
Acc. No. 88-317781/45 (Derwent Abstract) Sep. 28, 1988.
Acc. No. 95-188591/25 (Derwent Abstract) Apr. 21, 1995.
Acc. No. 95-218572/29 (Derwent Abstract) May 19, 1995.
Acc. No. 96-005215/01 (Derwent Abstract) Oct. 27, 1995.
|
Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Dahl; Philip Y.
Parent Case Text
This is a division of application Ser. No. 08/948,627 filed Oct. 10, 1997
now U.S. Pat. No. 5,910,378.
Claims
What is claimed is:
1. A method of producing an electrically conductive polymer film comprising
the step of heating a porous, polymer film comprising a polymer matrix and
about 45 to about 98 percent by weight electrically conductive particles
to a temperature within 20.degree. C. of the melting point of said polymer
matrix for sufficient time to decrease the Gurley value of said film by at
least about 25 percent and decrease the electrical resistivity of said
film by at least about 25 percent while substantially maintaining the
physical integrity and mechanical properties of said film upon cooling.
2. The method of claim 1, wherein said polymer matrix comprises a polymer
selected from the group consisting of polyethylene, polypropylene,
polyvinylidene fluoride, poly(tetrafluoroethylene-co-perfluoro-(propyl
vinyl ether)) and mixtures thereof.
3. The method of claim 1, wherein said conductive particles comprise
carbon.
4. The method of claim 1, wherein said conductive particles comprise one or
more conductive metals.
5. The method of claim 1, wherein said porous film comprises between about
80 and about 98 percent by weight conductive particles.
6. The method of claim 1, wherein said temperature is about 5 to about 20
degree centigrade above said melting temperature.
7. The method of claim 1, wherein said Gurley value of said film following
heating is less than 50 s/50 cc .
8. The method of claim 1, further comprising the step of differential
cooling for quenching the film to create an asymmetric film.
9. A method of producing a film comprising a polymer and greater than about
45 percent by weight conducting particles, said method comprising the
steps of heating to a temperature from about the melting point to about 20
degrees C. above the melting point and then stretching the film from about
25 percent to about 150 percent of their original length.
Description
FIELD OF THE INVENTION
The invention relates to membrane electrode assemblies and electrochemical
cells such as fuel cells, electrolyzers and electrochemical reactors.
BACKGROUND OF THE INVENTION
Fuel cells involve the electrochemical oxidation of a fuel and reduction of
an oxidizing agent to produce an electrical current. The two chemical
reactants, i.e., the fuel and the oxidizing agent, undergo redox reaction
at two isolated electrodes, each containing a catalyst in contact with an
electrolyte. An ion conduction element is located between the electrodes
to prevent direct reaction of the two reactants and to conduct ions.
Current collectors interface with the electrodes. The current collectors
are porous so that reactants can reach the catalyst sites.
Fuel cells produce current as long as fuel and oxidant are supplied. If
H.sub.2 is the fuel, only heat and water are byproducts of the redox
reactions in the fuel cell. Fuel cells have application wherever
electricity generation is required. Furthermore, fuel cells are
environmentally benign.
An electrolyzer involves the splitting of water into hydrogen and oxygen
using electricity. Similarly, an electrochemical reactor, such as a
chlor-alkali cell, uses electricity to produce chlorine from an alkaline
brine. Electrolyzers and electrochemical reactors basically involve a fuel
cell operating in reverse. For example, for an electrolyzer to produce
hydrogen and oxygen from water by passing an electrical current through
the device, an equivalent ion conductive element appropriate for use in a
fuel cell may be located between catalyst layers and current collector
layers.
SUMMARY OF THE INVENTION
In a first aspect, the invention features an electrochemical MEA
comprising:
an ion conductive membrane, the membrane having a first and second major
surface;
catalyst adjacent to the first and second major surfaces; and
a porous, electrically conductive polymer film adjacent to the ion
conductive membrane, the film comprising a polymer matrix and about 45 to
about 98 percent by weight electrically conductive particles embedded
within the polymer matrix.
In a preferred embodiment, the Gurley value of the polymer film is less
than about 50s/50cc. The polymer matrix can include a polymer selected
from the group consisting of polyethylene, polypropylene, polyvinylidene
fluoride, polytetrafluoroethylene,
poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) and mixtures
thereof. The electrically conductive particles can comprise carbon. The
porous polymer film preferably has an electrical resistivity of less than
about 20 ohm-cm.
The catalytic material can be disposed at an interface between the ion
conductive membrane and the porous, electrically conductive polymer film.
The catalytic material can be disposed upon the surfaces of the ion
conductive membrane. In preferred embodiments, the catalytic material is
disposed in nanostructured elements.
In another aspect, the invention features an electrochemical MEA
comprising:
an ion conductive membrane, the membrane having a first and second major
surface;
catalyst adjacent to the first and second major surfaces; and
a porous, electrically conductive polymer film adjacent to the ion
conductive membrane, the film comprising electrically conductive particles
and a porous matrix of fibrillated PTFE fibrils.
The catalytic material can be disposed at an interface between the ion
exchange membrane and the porous, electrically conductive polymer film.
The catalytic material can be disposed upon at least one major surface of
the electrically conductive polymer film. The conductive particles can
comprise carbon. The porous polymer film preferably has a Gurley value of
less than 50 s/50 cc and an electrical resistivity of less than 20 ohm-cm.
In another aspect, the invention features a method of producing an
electrically conductive polymer film comprising the step of heating a
porous, polymer film comprising a polymer matrix and about 45 to about 98
percent by weight electrically conductive particles to a temperature
within 20.degree. C. of the melting point of the polymer matrix for
sufficient time to decrease the Gurley value of the film by at least about
25 percent and decrease the electrical resistivity of the film by at least
about 25 percent while substantially maintaining the physical integrity
and mechanical properties of the film upon cooling. The polymer matrix can
include a polymer selected from the group consisting of polyethylene,
polypropylene, polyvinylidene fluoride,
poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) and mixtures
thereof. The conductive particles can comprise carbon and/or one or more
conductive metals. The porous film preferably includes between about 80
and about 98 percent by weight conductive particles. The temperature can
range between about 5 to about 20 degree centigrade above the melting
temperature. The Gurley value of the film following heating preferably is
less than 50 s/50 cc . The method can further comprise the step of using
differential cooling for quenching the extruded film to create an
asymmetric film with one side being denser and having smaller pores and
the other side being less dense and having larger pores. The differential
cooling can be accomplished through the use of a casting wheel at a
controlled temperature.
In another aspect, the invention features a method of forming an electrode
backing layer for an electrochemical MEA comprising the steps of:
(a) forming a polymeric film comprising a crystallizable polyolefin polymer
matrix, conductive particles and a diluent for the polymer;
(b) applying surface texture to the polymeric film; and
(c) removing the oil before or after applying the surface texture.
In another aspect, the invention features a method of forming an
electrochemical MEA comprising the step of placing an electrode backing
layer on both sides of a polymeric ion conductive membrane, the electrode
backing layers each comprising a gas permeable, electrically conductive
porous film prepared as described in the preceding paragraph, wherein a
catalyst layer is disposed between each of the ion conductive membrane and
the electrode backing layers.
In another aspect, the invention features a method of forming an
electrochemical MEA comprising the step of placing an electrode backing
layer on both sides of a polymeric ion conductive membrane, the electrode
backing layers each comprising a gas permeable, electrically conductive
porous fibrillated PTFE film and conductive particles embedded in the
film, wherein a catalyst layer is disposed between each of the ion
conductive membrane and the electrode backing layers.
In another aspect, the invention features a method of producing a plurality
of 5-layer MEAs, comprising the step of applying catalyst layers and
electrode backing layers at suitable locations along a web of ion
conduction membrane such that a plurality of 5-layer MEAs can be cut from
the web of ion conduction membrane.
In another aspect, the invention features a film comprising greater than
about 45 percent by weight conducting particles, the film having a surface
exhibiting under contact with water a receding and advancing contact
angles greater than 90.degree., wherein the advancing contact angle is no
more than 50.degree. greater than the receding contact angle. The
advancing contact angle preferably is no more than 30.degree. greater than
the receding contact angle, more preferably no more than 20.degree.
greater than the receding contact angle.
In another aspect, the invention features a method of producing a film
comprising a polymer and greater than about 45 percent by weight
conducting particles, the method comprising the steps of heating to a
temperature from about the melting point to about 20 degrees C. above the
melting point and then stretching the film from about 25 percent to about
150 percent of their original length.
In another aspect, the invention features a polymer web including a
plurality of MEA elements. The MEA elements can be disposed along a
continuous web of ion conducting polymeric material. The polymer web can
further include nanostructured catalyst layers and/or suitably located
seal material.
Electrode backing layers as described herein have high electrical
conductivity, high gas permeability, good water management characteristics
and significant production advantages. Membrane electrode assemblies
(MEAs) incorporating the electrode backing layers can have improved
performance as determined by the current produced at a given fuel cell
voltage. Advantageously, films of the present invention exhibit adequate
hydrophobicity for effective water management without incurring the
expense or the need for a fluoropolymer coating, whose properties can
change with use. The porous polymeric, electrode backing layers can be
used in efficient, commercial production methods of multilayer MEAs
including continuous roll processes. Continuous roll processing allows for
the cost effective assembly of hundreds of electrochemical cell components
at a relatively rapid rate.
Other features and advantages of the invention will be apparent from the
following description of the preferred embodiments thereof, and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross section of a five layer MEA.
FIGS. 2 and 2A are schematic cross sections of a fuel cell stack.
FIGS. 3 and 3A are perspective views of a continuous roll of MEAs.
FIG. 4 is an exploded, perspective view of a fuel cell stack with three
cells.
FIG. 5 is a graph depicting the phase behavior of a crystalline,
thermoplastic polymer, useful for evaluating the proper conditions in the
TIPT process.
FIG. 6 is a graph of cell voltage vs current density to obtain the
resistivity at high current density of carbon-loaded electrode backing
materials obtained using the TIPT process.
FIG. 7 is a graph of cell voltage versus current density for two five layer
MEAs incorporating electrode backing layers produced using the TIPT
process and, for comparison, a cell produced using commercial electrode
backing material.
FIG. 8 is a graph of cell voltage vs current density for additional cells
produced with carbon-loaded electrode backing materials obtained using the
TIPT process and, for comparison, a cell produced using commercial
electrode backing materials.
FIG. 9 is a bar graph showing the Gurley values for electrode backing
layers produced using the PF process and, for comparison, commercial
electrode backing material.
FIG. 10 is a graph of the applied voltage as a function of current density
for electrode backing layers produced using the PF process and a
commercial electrode backing material, as measured in a fuel cell test
assembly.
FIG. 11 is a graph of cell voltage versus current density for fuel cell
MEA's incorporating electrode backing layers produced using the PF process
compared to a fuel cell MEA produced with a commercial electrode backing
layer.
FIG. 12 is a graph of cell voltage versus current density for fuel cell
MEA's incorporating electrode backing layers produced using the TIPT
process along with a control incorporating a commercial material, each
tested with equivalent catalyst coated ion conduction membranes.
FIG. 13 is a graph of cell voltage versus current density for fuel cells
incorporating electrode backing layers produced using the TIPT process
using a smooth casting wheel showing the difference in cell performance
for an asymmetric electrode backing layer in the cell depending on the
orientation of the electrode backing film with respect to side-to-side
placement in the cell.
FIG. 14A is a SEM micrograph of the casting wheel side of the carbon-filled
HDPE film of Example 16A without heat treatment.
FIG. 14B is a SEM micrograph of the air side of the carbon-filled HDPE film
of Example 16A without heat treatment.
FIG. 14C is a SEM micrograph of a cross-section of the carbon-filled HDPE
film of Example 16A without heat treatment.
FIG. 15A is a SEM micrograph of the casting wheel side of the carbon-filled
HDPE film of Example 16B following heat treatment at 130.degree. C.
FIG. 15B is a SEM micrograph of the air side of the carbon-filled HDPE film
of Example 16B following heat treatment at 130.degree. C.
FIG. 15C is a SEM micrograph of the cross-section of the carbon-filled HDPE
film of Example 16B following heat treatment at 130.degree. C.
FIG. 16A is a SEM micrograph of a cross-section of the carbon-filled UHMWPE
film of Example 14 before heat treatment.
FIG. 16B is a SEM micrograph of a cross-section of the carbon-filled UHMWPE
film of Example 14 after heat treatment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A. Electrochemical Cell Structure
Referring to FIG. 1, membrane electrode assembly (MEA) 100 in a five layer
embodiment has various layers for the electrochemical oxidation of a fuel
and reduction of an oxidizing agent to produce electric current. An ion
conductive membrane 102 separates the cathode 104 and anode 106 of MEA
100. Each side of ion conductive membrane 102 contacts a catalyst layer,
i.e., cathode 104 and anode 106. Catalyst layers 104, 106 each contact an
electrode backing layer 108, 110. Electrode backing layers 108, 110
respectively contact bipolar plates 112, 114. The shape and size of the
components of the electrochemical cell can vary over a wide range
depending on particular design. FIG. 1 depicts the flow of reactants for a
fuel cell. In electrolyzers and electrochemical reactors, a voltage is
applied to the MEA to decompose a composition flowed to the electrodes,
for example for the formation of Cl.sub.2. The discussion below focuses on
fuel cells, although the analogy to electrolyzers and electrochemical
reactors is straightforward.
The ion conductive membrane provides ionic conductivity between the anode
and cathode and forms a gaseous barrier blocking flow of the reactants. In
some embodiments, the ion conductive membrane may be conductive only of
ions either of positive charge or negative charge, i.e., either a cation
exchange membrane or an anion exchange membrane, or only of one type of
ion, e.g., a proton exchange membrane.
While being conductive of some type of ions, the ion conductive membrane
should be nonconductive with respect to electrons and gaseous reactants.
To prevent the passage of gaseous reactants, the ion conductive membrane
should have sufficient thickness for mechanical stability and should be
effectively nonpermeable. The conduction of gaseous reactants through the
ion exchange membrane could result in the undesirable direct reaction of
the reactants. Similarly, the conduction of electrons through the ion
conductive membrane could result in an undesirable short circuit of the
cell. Therefore, materials used in producing the ion conductive membrane
should not conduct electrons. In the case of direct reaction of the
reactants or of a short circuit, the energy released by the reaction of
the fuel and oxidizing agent cannot be used to produce electricity.
The ion conductive membrane can include a polymer electrolyte. The polymers
should be chemically stable and compatible with the catalysts so that the
catalyst is not poisoned. Polymer electrolytes can be made from a variety
of polymers including, for example, polyethylene oxide, poly (ethylene
succinate), poly (.beta.-propiolactone), and sulfonated fluoropolymers
such as Nafion.TM. (commercially available from DuPont Chemicals,
Wilmington, Del.). Nafion.TM. is produced by hydrolyzing a copolymer of
polytetrafluoroethylene with perfluorosulfonylethoxyvinylether and
converting its sulfonyl radical to a sulfonic radical. A suitable cation
exchange membrane is described in U.S. Pat. No. 5,399,184, incorporated
herein by reference.
Alternatively, the ion conductive membrane can be an expanded membrane with
a porous microstructure where an ion exchange material impregnates the
membrane effectively filling the interior volume of the membrane. U.S.
Pat. No. 5,635,041, incorporated herein by reference, describes such a
membrane formed from expanded polytetrafluoroethylene (PTFE). The expanded
PTFE membrane has a microstructure of nodes interconnected by fibrils.
Similar structures are described in U.S. Pat. No. 4,849,311, incorporated
herein by reference.
The half-cell reactions of the fuel and the oxidizing agent take place at
separate catalyst surfaces. The reactant gases, i.e., fuel and oxidizing
agent, must be able to penetrate to their respective catalyst layer. A
catalyst generally is in the form of particles disposed in a layer with an
ionomer or electrolyte, in intimate contact with the ion conductive
membrane and the electrode backing layer. The catalyst layer can be
applied to the ion conductive membrane or the electrode backing layer by
various methods. In other words, the catalyst can be applied to the
surface of the ion conductive membrane and/or to a surface of the
electrode backing layer. Alternatively, the catalyst layer can be
encapsulated or embedded in the surface of the ion conductive membrane.
For example, the ion conductive membrane can include a nanostructured
catalyst layer such as the membranes described in U.S. Pat. No. 5,338,430,
incorporated herein by reference. The nanostructured films have a
plurality of nanostructured elements that are either two-component
whiskers coated with catalytically active material or one component
structures including catalytically active material. The nanostructured
elements can be embedded in an encapsulant such as a solid electrolyte, an
ion exchange membrane, or other polymeric matrix. The production of
nanostructured membranes is described in U.S. Pat. No. 5,238,729,
incorporated herein by reference.
Appropriate catalysts for fuel cells generally depend on the reactants
selected. Suitable catalyst materials for oxidation of hydrogen or
methanol fuels include metals such as, for example, Pd, Pt, Ru, Rh and
alloys thereof. Commonly used catalysts for oxygen reduction include
platinum supported on carbon particles. Different catalysts may be
preferred for use in electrolyzers and electrochemical reactors. For
example, for oxygen evolution in an electrolyzer, a mixture of Ru and Ir
oxides generally show better performance than Pt.
The electrode backing layer functions as a current collector. The electrode
backing layer is porous for the passage of gaseous reactants. To impart
electrical conductivity, the electrode backing layer includes electrically
conducting particles. If desired, the electrode backing layer can be
textured. Detailed features of the electrode backing layer are described
below.
Bipolar plates typically have channels and/or grooves in their surfaces
that distribute fuel and oxidant to their respective catalyst electrodes.
Typically, bipolar plates are highly electrically conductive and can be
made from graphite and metals. The electrodes and electrode backing layers
of the present invention generally can be used with any standard fuels
including H.sub.2 and reformed hydrocarbons such as methanol and gasoline,
and standard oxidants including O.sub.2 in air or in pure form.
Generally, a plurality of fuel cells or MEAs 150 are combined to form a
fuel cell stack 152 as depicted in FIG. 2. The cells within the stacks are
connected in series by virtue of the bipolar plates such that the voltages
of the individual fuel cells are additive. Further details relating to
formation of a fuel cell stack are presented below.
B. Electrode Backing Layer/Electrode
The electrode backing layer comprises a porous polymer film including a
polymer binder and conductive particles. In general, the film should have
a high loading of conductive particles held together by a relatively small
portion of polymer matrix. The film generally has greater than about 45
percent by volume conductive particles and more preferably between about
65 percent and about 96 percent by volume conductive particles. In
addition to the conductive particles, a catalyst layer (electrode) can be
coated on a surface of the electrode backing layer.
The porosity of the polymer film forming the electrode backing layer
provides for flow of reactants to the catalyst particles at the interface
of the electrode backing layer and the ion conductive membrane. Preferred
films have a porosity adequate to provide for an even flow of reactants
while maintaining adequate electrical conductivity and mechanical strength
of the film. Also, the porosity of the polymer film provides for water
management within the cell. The electrode backing layer preferably is
sufficiently porous to pass fuel gas and water vapor through it without
providing a site for water condensation that would block the pores of the
film and prevent vapor transport. The mean pore size generally ranges from
about 0.01 micrometers to about 10.0 micrometers. Alternatively, porosity
of the web can be quantified by the Gurley value of the web, that is, the
amount of time needed for a given volume of gas to pass through a
predetermined area of the web under a specified pressure drop, as
described below. Suitable webs generally have Gurley values less than
about 100 seconds per 10 cc.
To assist with water management, electrode backing layers with asymmetric
porosity can be used. The electrode backing layer adjacent to the cathode,
where water is formed, preferably has smaller pores adjacent to the
cathode and larger pores at the outside of the MEA adjacent the bipolar
plate. The higher pressure in the small pores tends to push the water away
from the cathode. The formation of electrode backing layers having
asymmetric porosity is described below.
Conductive particles can include a variety of conductive materials such as
metals and carbon. The conductive particles can have a variety of shapes
and sizes. Preferred conductive particles include, for example, conductive
carbons. The conductive particles are preferably less than about 10
microns in diameter and more preferably less than about 1 micron in
diameter. Suitable carbon particles include, for example, carbon black,
graphite, carbon fibers, fullerenes and nanotubules. Preferred carbon
particles include, for example, carbon blacks. Commercially available
carbon blacks include, for example, Vulcan XC72R.TM. (Cabot Corp.,
Bilerica, Mass.), Shawinigan C-55.TM. 50% compressed acetylene black
(Chevron Chemical Co., Houston, Tex.), Norit type SX1.TM. (Norit Americas
Inc., Atlanta, Ga.), Corax L.TM. and Corax P.TM. (Degussa Corp.,
Ridgefield Park, N.J.), Conductex 975.TM. (Colombian Chemical Co.,
Atlanta, Ga.), Super S.TM. and Super P.TM. (MMM Carbon Div., MMM nv,
Brussels, Belgium), KetJen Black EC 600JD.TM. (Akzo Nobel Chemicals, Inc.,
Chicago, Ill.). Useful graphite particles range in size up to about 50
.mu.m in diameter, preferably from about 1 to about 15 .mu.m. Suitable
commercial graphites include, for example, MCMB 6-28.TM. (Osaka Gas
Chemical Co., Osaka, Japan), and SFG 15.TM. (Alusuisse Lonza America Inc.,
now Timcal, Fair Lawn, N.J.). Conductive carbon black can have primary
particles as small as about 10 nm to about 15 nm, though as sold they may
be present in agglomerates as large as several mm. After dispersion, these
agglomerates are broken down preferably into particles less than about 0.1
micron (100 nm). Mixtures of graphite and more conductive carbon blacks
are also useful. Conductive carbon fibers useful in electrode backing
materials of the invention include, e.g., those available from STREM
Chemicals, Inc., Newburyport, Mass., catalog No. 06-0140, having lengths
of approximately 6 mm and diameters of 0.001 cm.
In general, the polymer matrix can include any polymer that can be
processed appropriately into a porous film loaded with particles. Suitable
types of polymers include, for example, thermoplastic polymers,
thermosensitive polymers and fluoropolymers. Two preferred processing
methods are described below. These preferred processing methods provide
additional constraints on the characteristics of the corresponding
polymers.
In addition to the conductive particles, fillers can be used to alter the
physical properties of the polymer films useful in the invention.
Appropriate fillers include, e.g. silica (SiO.sub.2), powdered
polytetrafluoroethylene and graphite fluoride (CF.sub.n). The polymer
films preferably can include up to about 20 percent by weight fillers, and
more preferably from about 2 to about 10 percent by weight fillers. The
fillers are generally in the form of particles.
Preferably, the electrode backing layers have an electrical resistivity of
less than about 20 Ohm-cm, more preferably less than about 10 Ohm-cm, and
most preferably less than about 0.5 Ohm-cm. Also, films useful as
electrodes in the invention preferably exhibit advancing and receding
contact angles toward water of greater than about 90.degree., more
preferably of greater than about 110.degree. wherein the advancing contact
angle is greater than the receding contact angle by less than about
50.degree., preferably less than about 30.degree., and more preferably
less than about 20.degree.. The measurement of the advancing and receding
contact angles is described below. Receding and advancing contact angles
of water are an important measure of the hydrophobicity of the film
surface and the ability of the film to function effectively in the water
management of the fuel cell. The contact angles can be different on the
two surfaces of the electrode backing layer. Similarly, the contact angles
for the cathode and anode can be different.
The resistance to gas flow of a polymer film can be expressed in terms of
the Gurley value. The Gurley value is a measure of the flow rate of a gas
through a standardized area of the film under controlled pressure
conditions, as described in ASTM D726-58, Method A, as described further
below. The electrode backing layers preferably have a Gurley value of less
than about 100 sec/50 cc air and more preferably less than about 50 sec/50
cc air.
The surfaces of the electrode backing layers can be microtextured possibly
providing enhanced interfacial electrical conductivity, water management
and flow field performance. For example, the material can be cast onto a
textured casting wheel or can be embossed using a nip roll wherein one of
the rolls is textured. A surface textured electrode backing layer can
facilitate gas (e.g. fuel, oxygen, and/or water vapor) transport into and
out of the fuel cell and channeling of liquid water away from the cathode.
Two processes for the production of preferred polymer films are described
next.
1. TIPT Process
The first preferred process for the production of porous electrode backing
layers involves thermally induced phase transition (TIPT). The TIPT
process is based on the use of a polymer that is soluble in a diluent at
an elevated temperature and insoluble in the diluent at a relatively lower
temperature. The "phase transition" can involve a solid-liquid phase
separation, a liquid-liquid phase separation or a liquid to gel phase
transition. The "phase transition" need not involve a discontinuity in a
thermodynamic variable.
Suitable polymers for the TIPT process include thermoplastic polymers,
thermosensitive polymers and mixtures of polymers of these types, with the
mixed polymers being compatible. Thermosensitive polymers such as
ultrahigh molecular weight polyethylene (UHMWPE) cannot be melt-processed
directly but can be melt processed in the presence of a diluent or
plasticizer that lowers the viscosity sufficiently for melt processing.
Suitable polymers may be either crystallizable or amorphous.
Suitable polymers include, for example, crystallizable vinyl polymers,
condensation polymers and oxidation polymers. Representative
crystallizable vinyl polymers include, for example, high and low density
polyethylene; polypropylene; polybutadiene; polyacrylates such as
polymethyl methacrylate; fluorine-containing polymers such as
polyvinylidene fluoride; and corresponding copolymers. Condensation
polymers include, for example, polyesters such as polyethylene
terephthalate and polybutylene terethphalate; polyamides such as nylons;
polycarbonates; and polysulfones. Oxidation polymers include, for example,
polyphenylene oxide and polyether ketones. Other suitable polymers include
the copolymer, poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether))
sold as Teflon.TM. PFA (E. I. DuPont de Nemours Chemical Corp.,
Wilmington; Del.). Blends of polymers and copolymers may also be used.
Preferred crystallizable polymers for electrode backing layers include
polyolefins and fluoropolymers, because of their resistance to hydrolysis
and oxidation.
Suitable diluents are liquids or solids at room temperature and liquids at
the melting temperature of the polymer. Low molecular weight diluents are
preferred since they can be extracted more readily than higher molecular
weight diluents. Low to moderate molecular weight polymers, however, can
be used as diluents if the diluent polymer and the matrix polymer are
miscible in the melt state. Compounds with boiling points below the
melting temperature of the polymer can be used as diluents by using a
superatmospheric pressure sufficient to produce a liquid at the polymer
melting temperature.
The compatibility of the diluent with the polymer can be evaluated by
mixing the polymer while heating to determine whether a single liquid
phase is formed, as indicated generally by existence of a clear
homogeneous solution. An appropriate polymer dissolves or forms a single
phase with the diluent at the melting temperature of the polymer but forms
a continuous network on cooling to a temperature below the melting
temperature of the polymer. The continuous network is either a separate
phase from the diluent or a gel where the diluent acts as a plasticizer
swelling the polymer network. The gel state may be considered a single
phase.
For non-polar polymers, non-polar organic liquids generally are preferred
as a diluent. Similarly, polar organic liquids generally are preferred
with polar polymers. When blends of polymers are used, preferred diluents
are compatible with each of the polymers. When the polymer is a block
copolymer, the diluent preferably is compatible with each polymer block.
Blends of two of more liquids can be used as the diluent as long as the
polymer is soluble in the liquid blend at the melt temperature of the
polymer, and a phase transition with the formation of a polymer network
occurs upon cooling.
Various organic compounds are useful as a diluent, including compounds from
the following broad classifications: aliphatic acids; aromatic acids;
aliphatic alcohols; aromatic alcohols; cyclic alcohols; aldehydes; primary
amines; secondary amines; aromatic amines; ethoxylated amines; diamines;
amides; esters and diesters such as sebacates, phthalates, stearates,
adipates and citrates; ethers; ketones; epoxy compounds such as epoxidized
vegetable oils; phosphate esters such as tricresyl phosphate; various
hydrocarbons such as eicosane, coumarin-indene resins and terpene resins,
tall oil, linseed oil and blends such as petroleum oil including
lubricating oils and fuel oils, hydrocarbon resin and asphalt; and various
organic heterocyclic compounds.
Examples of particular blends of polymers and diluents that are useful in
preparing suitable porous materials include polypropylene with aliphatic
hydrocarbons such as mineral oil and mineral spirits, esters such as
dioctyl phthalate and dibutyl phthalate, or ethers such as dibenzyl ether;
ultrahigh molecular weight polyethylene with mineral oil or waxes; high
density polyethylene with aliphatic hydrocarbons such as mineral oil,
aliphatic ketones such as methyl nonyl ketone, or an ester such as dioctyl
phthalate; low density polyethylene with aliphatic acids such as decanoic
acid and oleic acid, or primary alcohols such as decyl alcohol;
polypropylene-polyethylene copolymer with mineral oil; and polyvinylidene
fluoride with dibutyl phthalate.
A particular combination of polymer and diluent may include more than one
polymer and/or more than one diluent. Mineral oil and mineral spirits are
each examples of a diluent being a mixture of compounds since they are
typically blends of hydrocarbon liquids. Similarly, blends of liquids and
solids also can serve as the diluent.
For thermoplastic polymers, the melt blend preferably includes from about
10 parts to about 80 parts by weight of the thermoplastic polymer and from
about 90 to about 20 parts by weight of the diluent. Appropriate relative
amounts of thermoplastic polymer and diluent vary with each combination.
For UHMWPE polymers, an example of a thermosensitive polymer, the melt
blend preferably includes from about 2 parts to about 50 parts of polymer
and from about 98 parts to about 50 parts by weight of diluent.
For crystalline polymers the polymer concentration that can be used for a
solid-liquid or liquid-liquid phase separation in a given system can be
determined by reference to the temperature-composition graph for a
polymer-diluent system, an example of which is set forth in FIG. 5. Such
graphs can be readily developed as described in Smolders, van Aartsen and
Steenbergen, Kolloid-Zu Z. Polymere, 243:14-20 (1971). Phase transitions
can be located by determining the cloud point for a series of compositions
at a sufficiently slow rate of cooling that the system stays near
equilibrium.
Referring to FIG. 5, the portion of the curve from gamma to alpha
represents the thermodynamic equilibrium liquid-liquid phase separation.
T.sub.ucst represents the upper critical temperature of the systems. The
portion of the curve from alpha to beta represents the equilibrium
liquid-solid phase separation. The diluent can be chosen such that the
crystallizable polymer and diluent system exhibits liquid-solid phase
separation or liquid-liquid phase separation over the entire composition
range.
.PHI..sub.ucst represents the critical composition. To form the desired
porous polymers, the polymer concentration utilized for a particular
system preferably is greater than .PHI..sub.ucst. If the polymer
concentration is below the critical concentration (.PHI..sub.ucst), the
phase separation, upon cooling, generally forms a continuous phase of
diluent with dispersed or weakly associated polymer particles, and the
resulting polymer composition typically lacks sufficient strength to be
useful.
For a given cooling rate, the temperature-concentration curve of the
diluent-polymer blend can be determined by Differential Scanning
Calorimetry (DSC), for example, as indicated by the dashed line of FIG. 5
for one rate of cooling. The resulting plot of polymer concentration
versus melting temperature shows the concentration ranges that result in
solid-liquid (sloped portion of the dashed-curve) and in liquid-liquid
(horizontal portion of the dashed curve) phase separation. From this
curve, the concentration ranges of the polymer and liquid that yield the
desired porous structure can be estimated. The determination of the
melting temperature-concentration curve by DSC is an alternative to
determination of the equilibrium temperature-composition curve for a
crystalline polymer.
The above discussion of phase diagrams is applicable to amorphous polymers
except that only liquid-liquid phase separation can be observed. In this
case, a cloud point generally is indicative of the particular phase
transition. Similarly, for gel forming polymers the phase transition of
relevance involves a transition from a homogeneous solution to a gel. With
gel forming polymers, an abrupt increase in viscosity is indicative of a
phase transition from the melt to the gel, although a cloud point may also
occur in some cases.
For many diluent-polymer systems, when the rate of cooling of the
liquid-polymer solution is slow, liquid-liquid phase separation occurs at
substantially the same time as the formation of a plurality of liquid
droplets of substantially uniform size. When the cooling rate is slow
enough such that the droplets form, the resultant porous polymer has a
cellular microstructure. In contrast, if the rate of cooling of the
liquid-polymer solution is rapid, the solution undergoes a spontaneous
transformation called spinodal decomposition, and the resultant porous
polymer has a fine, lacy structure with a qualitatively different
morphology and physical properties than obtained following droplet
formation, which can be obtained if the rate of cooling is slow. The fine
porous structure is referred to as a lacy structure
When liquid-solid phase separation occurs, the material has an internal
structure characterized by a multiplicity of spaced, randomly disposed,
non-uniform shaped, particles of polymer. Adjacent polymer particles
throughout the material are separated from one another to provide the
material with a network of interconnected micropores and being connected
to each other by a plurality of fibrils consisting of the polymer. The
fibrils elongate upon orientation providing greater spacing between the
polymer particles and increased porosity. The filler particles reside in
or are attached to the thermoplastic polymer of the formed structure.
In the case of ultrahigh molecular weight polyethylene (UHMWPE), the
article obtained upon cooling may exist in a gel state. The nature of the
underlying polymer network is affected by the rate of cooling. Fast
cooling tends to promote gel formation while slower cooling tends to allow
more crystallization to occur. Gel formation tends to dominate for
compositions having diluent UHMWPE weight ratios greater than 80:20,
whereas crystallization dominates increasingly for diluent/UHMWPE weight
ratios less than 80:20. The polymer network in the case of highly particle
filled UHMWPE as determined by SEM, after extraction of the diluent, tends
to be a fairly dense structure having fine pores. The structure of the
network can be changed by the extraction process. The highly particle
filled UHMWPE films are porous after extraction without need for restraint
during extraction or stretching.
If desired, the polymer can be blended with certain additives that are
soluble or dispersible in the polymer. When used, the additives are
preferably less than about 10 percent by weight of the polymer component
and more preferably less than about 2 percent by weight. Typical additives
include, for example, antioxidants and viscosity modifiers.
The melt blend further includes particulates for incorporation into the
electrode. For the resulting filled compositions, porous polymer films can
be obtained by extraction of the diluent without physical restraint during
extraction or stretching of the film. In some cases, restraint of the film
during extraction may result in larger bubble points and smaller Gurley
values than for a film extracted without restraint. Particles for the
production of an electrode backing layer can include conductive particles.
The particles can be a mixture of materials. The particles preferably form
a dispersion in the diluent and are insoluble in the melt blend of polymer
and diluent. The appropriate types of materials have been described above,
as long as the materials are appropriately compatible with the polymer and
diluent.
Some of the particulates, especially small sized carbon particles, can
serve as nucleating agents. The nucleating agent can be a solid or gel at
the crystallization temperature of the polymer. A wide variety of solid
materials can be used as nucleating agents, depending on their size,
crystal form, and other physical parameters. Smaller solid particles,
e.g., in the submicron range, tend to function better as nucleating
agents. Preferably, nucleating agents range in size from about 0.01 to
about 0.1 .mu.m and more preferably from about 0.01 to about 0.05 .mu.m.
Certain polymers such as polypropylene perform better in the TIPT process
with a nucleating agent present.
In the presence of a nucleating agent, the number of sites at which
crystallization is initiated is increased relative to the number in the
absence of the nucleating agent. The resultant polymer particles have a
reduced size. Moreover, the number of fibrils connecting the polymer
particles per unit volume is increased. The tensile strength of the
material is increased relative to porous films made without the nucleating
agent.
In the porous networks, preferably the particles are uniformly distributed
in the polymer matrix, and are firmly held in the polymeric matrix such
that they do not wash out on subsequent extraction of the diluent using
solvent. The average particle spacing depends on the volume loading of the
particles in the polymer, and preferably, in the case of conductive
particles, the particles are in sufficiently close proximity to sustain
electrical conductivity. Processing of particles in the polymer matrix,
particularly conductive carbon particles, requires care, since undermixing
can result in poor dispersion, characterized by lumps of particles (e.g.,
knots of carbon), and overmixing can cause the agglomerates to disperse
completely in the polymer. Conductive particle proximity is important for
higher levels of conductivity. Therefore, both extremes are unfavorable
for the conductive properties of the mixture.
The melt blend can contain as high as about 40 percent to about 50 percent
by volume dispersed particles. By combining high diluent concentrations
with high volume percent of particles, a high weight percent of particles
can be achieved after the diluent has been extracted from the phase
separated polymer composition. Preferably, the extracted and dried polymer
material includes from about 50 percent to about 98 percent particles and
more preferably from about 70 percent to about 98 percent by weight
particles.
The diluent eventually is removed from the material to yield a
particle-filled, substantially liquid-free, porous electrically-conductive
polymeric material. The diluent may be removed by, for example, solvent
extraction, sublimation, volatilization, or any other convenient method.
Following removal of the diluent, the particle phase preferably remains
entrapped to a level of at least about 90 percent, more preferably about
95 percent and most preferably about 99 percent, in the porous structure.
In other words, few of the particles are removed when the diluent is
eliminated, as evidenced by lack of particulates in the solvent washing
vessel.
The process is described below generally and can be varied based on the
teachings herein. In one embodiment of the TIPT process, the particles are
disposed beneath the surface of the diluent, and entrapped air is removed
from the mixture. A standard high speed shear mixer operating at several
hundred RPM to several thousand RPM for about several minutes to about 60
minutes can be used to facilitate this step. Appropriate high speed shear
mixers are made, for example, by Premier Mill Corp., Reading, Pa. and by
Shar Inc., Fort Wayne, Ind.
If more dispersion is needed following the first mixing step, it can be
achieved through milling of the dispersion before pumping the dispersion
into the extruder, or through introduction of dispersing elements into the
extruder. For shear sensitive polymers such as UHMWPE, most particulate
dispersion preferably is done prior to pumping the dispersion into the
extruder to minimize the shear needed in the extruder. If required, the
second step involves dispersing the particles in the diluent and may
include breaking down particle agglomerates to smaller agglomerates to
eliminate large clumps within the diluent. Complete dispersion to primary
particles generally is not necessary or desirable since contact or
proximity between conducting particles generally promotes electrical
conductivity.
The degree of preferred dispersion can be determined by inspection of the
final electrode film for surface roughness and by determining its
conductivity. The surface should be generally smooth and uniform with no
protrusions through the surface large enough to be seen by eye.
Insufficient dispersion of particulates can result in films having rough
surfaces with a texture of fine to coarse sandpaper. In certain instances,
no milling is needed since the shear used simply to wet out the
particulates results in sufficient dispersion. Appropriate selection of
components such as the diluent and the initial particles can greatly
facilitate the dispersing step.
When additional dispersion is required or desired, the diluent containing
the particulate material can be processed in a mill. Preferably,
particle/diluent milling is carried out at relatively high viscosity where
the milling process is more effective. Useful mills include, for example,
attritors, horizontal bead mills and sand mills. Typically, a single pass
through a horizontal bead mill at a moderate through-put rate (i.e.,
moderate relative to the maximum through-put rate of the mill) is
sufficient. When significant amounts of dispersion are required, milling
times for recirculation of the dispersion through the mill of less than an
hour may be sufficient in some cases, while milling times of at least
about 4 to about 8 hours may be needed in other cases.
An example of an appropriate instrument for processing small batches is an
attritor Model 6TSG-1-4, manufactured by Igarachi Kikai Seizo Co. Ltd.,
Tokyo, Japan. This attritor has a water-cooled with about a 1 liter volume
which operates at about 1500 RPM with a capacity to process about 500 cc
of material. For larger batches, appropriate instruments include
horizontal mills such as those sold by Premier Mill Corp., Reading, Pa.,
in a variety of sizes.
Milling reduces agglomerates to smaller agglomerates or primary particles
but generally does not break down primary particles to smaller particles.
Filtration of the milled dispersion may be an optional step, if a greater
number of larger particles are present than desired. An appropriate filter
would be, for example, a model C3B4U 3 micron rope-wound filter made by
Brunswick Technitics (Timonium, Md.) to remove agglomerated particles or
particles larger than 3 microns, for example.
Filtering results in a more uniform article and allows metering of the
dispersions under pressure by close tolerance gear pumps during the
extrusion process without frequent breakdowns due to large particles
clogging the pump. After filtering, the concentration of the particles can
be determined, for example, using a Model DMA-4S Mettler/Paar density
meter manufactured by Mettler-Toledo, Inc., Hightstown, N.J.
A dispersant can be added to the mixture of diluent and particles to aid in
stabilizing the dispersion of particles in the diluent and in maintaining
the particles as unaggregated. If a dispersant is used, the
diluent-particle mixture preferably contains from about 1 percent to about
100 percent by weight of dispersant relative to the weight of the
particles.
Anionic, cationic and nonionic dispersants can be used. Examples of useful
dispersants include OLOA 1200.TM., a succinimide lubricating oil additive,
available from Chevron Chemical Co., Houston, Tex., or the Hypermer.TM.
series of dispersants, available from ICI Americas, Wilmington, Del.
The diluent-particle mixture generally is heated to about 150.degree. C. to
degas the mixture before pumping the mixture into an extruder. The mixture
can be pumped into the extruder with or without cooling the mixture to
ambient temperature. The polymer is fed typically into the feed zone of
the extruder using a gravimetric or volumetric feeder. (In an alternative
embodiment, at least some of the carbon is fed with the polymer into the
extruder.) For thermoplastic polymers, feed and melt zone temperatures
preferably are selected so that the polymer is at least partially melted
before contacting diluent. If the particles are easily dispersed, the
particles can be fed at a controlled rate into the extruder, and the
diluent separately metered into the extruder. Also, a variety of in-line
mixers are available that provide for dispersion of particulates on a
continuous in-line basis from streams of particles and liquids.
Alternatively, in cases where adequate dispersion can be obtained in the
extruder, separate streams of polymer, diluent and conductive particles
can be fed directly into the extruder.
Then, a melt blend of the diluent-particle mixture is formed with the
polymer in the extruder. Following sufficient mixing in the extruder, the
melt blend is cast into the desired form. Typically, since a film is
desired, the melt blend is extruded onto a temperature-controlled casting
wheel using a drop die. A twin-screw extruder is preferred.
Following formation of the desired shape of material, the material is
cooled, preferably rapidly, to induce the phase transition. Quench
conditions depend on film thickness, extrusion rate, polymer composition,
polymer-to-diluent ratio, and desired film properties. Preferred
conditions for a specific film can be readily determined. For higher
quench temperatures, film strength may be diminished relative to films
formed at lower quench temperatures. Rapid cooling can be accomplished by,
for example, cooling in sufficiently cold air, cooling by contact on one
or more sides with a temperature-controlled casting wheel or immersion of
the material in a temperature-controlled liquid. Following quenching, the
diluent is removed. If solvent is used to remove the diluent, remaining
solvent is removed by evaporation.
For a given polymer-diluent combination, use of a casting wheel, especially
a smooth casting wheel, can result in an asymmetric film. As the casting
wheel temperature is lowered, it is increasing likely that the resulting
film will be asymmetric. Typically, the side of the film toward the
casting wheel has a "skin" that is denser and has smaller pores.
Alternatively, a higher casting wheel temperature relative to the air
temperature can result in a denser surface layer on the air side. In
general, a lower casting wheel temperature produces a film that is
stronger, denser on the casting wheel side, and has a smaller bubble point
and higher Gurley value. Asymetric films can be produced by other
asymmetric quenching methods.
2. Polymer-Fibrillation (PF) Process
The second preferred process for the formation of porous electrode backing
layers involves the preparation of a porous web comprising conductive
particles, such as carbon, metals, and the like, enmeshed in a fibril
forming polymer. The process includes the formation of a mixture of the
fibril forming polymer, a lubricant and insoluble nonswellable particles
such as conductive carbon particles. The particles are approximately
evenly distributed in the composite and are enmeshed in the fibril forming
polymer. This process is adapted from the process outlined in U.S. Pat.
Nos. 4,153,661, 4,460,642, 5,071,610, 5,113,860, and 5,147,539, which are
incorporated herein by reference.
Preferred fibril forming polymers include halogenated vinyl polymers such
as polytetrafluoroethylene (PTFE). Dry powder PTFE such as Teflon.TM.6C
can be used as the starting material. Alternatively, the process can be
performed starting with a commercially-available aqueous dispersion of
PTFE particles, such as Teflon 30.TM., Teflon 30b.TM. and Teflon 42.TM.
(E. I. DuPont de Nemours Chemical Corp., Wilmington, Del.), wherein water
acts as a lubricant for subsequent processing. Commercially available PTFE
aqueous dispersions may contain other ingredients such as surfactants and
stabilizers, which promote continued suspension of the PTFE particles. In
some applications, it is advantageous to remove the surfactant, if
present, by extraction at a desired point in the process.
The lubricant must be selected such that the polymer is not soluble in the
lubricant. Preferred lubricants include water, organic solvents and
mixtures of water and miscible organic solvents that can be conveniently
removed by washing or drying. In some circumstances water has a
deleterious effect on the added particles (i.e., causes unacceptable
swelling or agglomeration) or inhibits dispersion of the particles.
Suitable organic lubricants include, for example, alcohols, ketones,
esters, ethers, and fluorinated fluids. Fluorinated fluids include, for
example, perfluorinated compounds such as Fluorinert.TM. (3M, Saint Paul,
Minn.) or other competitive perfluorinated compositions. "Perfluorinated"
is used to indicate that substantially all of the hydrogen atoms have been
replaced by fluorine atoms. Electrode backing layers containing carbon
particles preferably are prepared using a perfluorinated liquid lubricant.
Preferably, the liquid used is Fluorinert FC-40.TM., although other
liquids such as Fluorinert FC-5312.TM. can also be used. Alternatives also
include Galden.TM. and Fomblin.TM. perfluorinated fluids (Ausimont USA,
Thorofare, N.J.; Ausimont S.p.A., Montedison Group, Milan, Italy).
Preferred nonpolymer particles have a solubility of less than about 1.0
gram in 100 grams of lubricant at the mixing temperature. The particles
can be but do not need to be absorbent or adsorbent with respect to the
lubricant. The absorptive or adsorptive capability of the particles with
respect to lubricant preferably is less than about 10 percent by weight
and more preferably less than about 1 percent. The particles preferably
have an average diameter less than about 200 microns, more preferably in
the range from about 0.01 microns to about 100.0 microns and more
preferably in the range from about 0.1 microns to about 10.0 microns.
Generally, the nonpolymer particles are primarily or exclusively
conductive particles such as conductive carbon particles. Due to the
wetting properties of certain particles including conductive carbon
particles, non-aqueous, organic lubricants are preferred when the
particles are used in large quantities.
Small amounts of additives such as various particulate surface property
modifiers can be added. Any additional additives should be inert under the
conditions of operation of the fuel cell. Suitable additives include
synthetic and natural polymers such as polyethylene and polypropylene.
For electrode backing layers formed by the FP process, the weight ratio of
particles to polymer can be in the range from about 40:1 to about 1:4,
preferably from about 25:1 to about 1:1, and more preferably from about
20:1 to about 10:1. The lubricant preferably is added in an amount
exceeding the absorptive and adsorptive capability of the particles by at
least about 3 percent by weight and below an amount at which the polymer
mass loses its integrity, more preferably by at least about 5 weight
percent and less than about 200 percent, even more preferably by at least
about 25 percent and less than about 200 percent and yet even more
preferably by at least 40 percent and less than about 150 percent. In one
preferred embodiment, about 95 parts by weight of conductive particles is
used with about 5 parts by weight of PTFE, and the weight ratio of inert
fluid to solids (conductive particles plus PTFE) is about 8:1.
The absorptive capacity of the particles is exceeded when small amounts of
lubricant can no longer be incorporated into the putty-like mass without
separation of lubricant. A large viscosity change takes place
corresponding to a transition from a paste to a slurry. An amount of
lubricant exceeding the absorptive and adsorptive capacity of the
particles should be maintained throughout the entire mixing operation.
Since the void volume and porosity are controlled by the amount of
lubricant used, the amount of lubricant can be varied in order to obtain
electrode backing layers having a desired porosity and void volume.
Generally, increasing the amount of lubricant increases void volume and
mean pore size.
The mean pore size of the final article generally is in the range from
about 0.01 micrometers to about 10.0 micrometers, and more preferably from
about 0.1 micrometers to about 1.0 micrometers. With respect to
distribution of pore size, preferably at least about 90 percent of the
pores have a size less than 1 micrometer. The void volume as measured by
Mercury Intrusion Porosity preferably ranges from about 10 percent to
about 50 percent and more preferably from about 25 percent to about 35
percent. Typical Gurley values for webs of the invention range from about
2 seconds per 10 cc to about 100 seconds per 10 cc. Preferably, webs
useful in the invention exhibit a Gurley values of less than about 50
seconds per 10 cc and, more preferably less than about 40 seconds per 10
cc.
The resistivity of the final article generally is in the range from about
0.01 ohm-cm to about 10 ohm-cm, and more preferably from about 0.1 ohm-cm
to about 2.0 ohm-cm.
To practice the PF process, the materials are blended together to form a
soft dough-like mixture. If a solid powdered polymer is used, a low
surface energy solvent, as described above, can be used to disperse the
polymer into the mix. The blend is mixed at a temperature and for a time
sufficient to cause initial fibrillation of the PTFE particles. The mixing
temperature is selected to maintain the solvent in liquid form. The
temperature preferably is in the range from about 0.degree. C. and about
100.degree. C., preferably from about 20.degree. C. and about 60.degree.
C.
Initial fibrillation can take place simultaneously with the initial mixing
of the ingredients. If additional mixing is needed, mixing times generally
range from about 0.2 minutes to about 2 minutes to obtain initial
fibrillation of the fibril forming polymer. Initial fibrillation generally
is optimum within about 90 seconds after the point when all components
have been fully incorporated together into a putty-like consistency.
Mixing for shorter or longer times may produce a composite sheet with
inferior properties. Preferably, mixing is ended after going through or
reaching a viscosity maximum. This initial mixing causes partial
disoriented fibrillation of the fibril forming polymer particles.
Devices useful for obtaining the necessary intensive mixing include
commercially available mixing devices that sometimes are referred to as
internal mixers, kneading mixers, double-blade batch mixers, intensive
mixers and twin screw extruder compounding mixers. Preferred mixers of
this type include sigma-blade mixers and sigma-arm mixers. Commercially
available mixers of this type include those sold under the designations
Banbury.TM. mixer (Farrel Corp., Ansonia, Conn.), Mogul.TM. mixer
(Littelford Day Inc., Florence, Ky.), Brabender Prep.TM. mixer and
Brabender.TM. sigma blade mixer (C. W. Brabender Instruments, Inc., South
Hackensack, N.J.) and Ross.TM. mixers (AHing-Lander Co., Chesaire, Conn).
Following mixing, the putty-like mass is transferred to a calendering
device. The blend is subjected to repeated biaxial calendering between
calendering rolls to cause additional fibrillation of the polymer. For
typical lubricant/polymer combinations, the calendering rolls preferably
are maintained at a temperature less than about 125.degree., more
preferably at a temperature from about 0.degree. C. to about 100.degree.
C. and even more preferably from about 20.degree. C. to about 60.degree.
C. Lubricant lost to evaporation can be replaced between passes through
the calender. During calendering, lubricant levels are maintained at least
at a level exceeding the absorptive capacity of the solids by at least
about 3 percent by weight, until sufficient fibrillation occurs to produce
the desired void volume and porosity.
The calendering is repeated to form a self supporting tear resistant sheet.
The gap between the calendering rolls generally is decreased with each
successive pass. The material typically but not necessarily is folded and
rotated 90.degree. between passes through the calender. The number of
folds and gap settings can be adjusted to yield the desired properties of
the resultant sheet. As the calendering is repeated, the tensile strength
reaches a maximum beyond which additional calendering becomes deleterious.
Calendering generally is stopped after the maximum tensile strength is
reached and before the tensile strength deteriorates below the minimum
acceptable tensile strength. Generally, about 10 to about 20 passes
through the calendering rolls are appropriate. Once a web of the desired
thickness has been obtained, it can be air-dried at room temperature or
placed in a convection oven at an appropriate temperature in order to
remove excess inert fluid. Webs preferably have a final thickness in the
range of 0.1 to 1.0 mm, more preferably 0.2 to 0.5 mm, and even more
preferably in the range of 0.25 to 0.4 mm.
The resultant electrode backing layer preferably has a tensile strength of
at least about 1 megapascals and more preferably at least about 3
megapascals. The sheets are substantially uniformly porous with particles
generally uniformly distributed in a polymer fibril matrix. Almost all of
the particles are separated from each other yet the particles remain in
sufficient proximity such that good electrical conductivity is obtained.
C. Additional Processing
It has been discovered that the performance characteristics of
particle-loaded electrode backing layers, especially those produced with
the TIPT process, can be significantly improved by additional processing
once the polymer films are formed. First, the polymer electrode backing
layer can be heated to a temperature near the melting point of the polymer
matrix. The temperature preferably is in the range from about 20.degree.
C. above to about 20.degree. C. below the melting point of the polymer
matrix, more preferably at a temperature between the melting point and
10.degree. C. above the melting point.
Preferably, the heating is performed for a period of time to heat the
polymer electrode up to the target temperature and for polymer flow to
occur. For laboratory evaluation, a period of about 10 minutes is
sufficient to ensure that the film has equilibrated at the temperature of
the oven and for polymer flow to occur. This period of time accommodates
the inevitable loss of heat from an oven and time for the oven to
equilibrate at its set point. For continuous in-line processing, much
shorter residence times may be sufficient to allow enough time for heating
to the target temperature and for polymer flow to occur. Surprisingly,
this heating does not destroy the porosity of the film, even with the film
being unrestrained during heating. This heating step significantly reduces
the electrical resistance in the electrode backing layer while decreasing
the Gurley and increasing the bubble point value.
In addition, the electrode backing layers can be stretched. Depending on
the polymer, stretching generally can be carried out effectively at a
temperature from room temperature to about 20.degree. C. below the melting
point of the polymer, as determined by DSC. For highly particle filled
films, stretching is preferably carried out after extraction of the
diluent at temperatures within plus or minus 20 degrees C. of the melting
point of the polymer, as determined by DSC. Temperatures in this range
would normally result in loss of porousity for unfilled films with the
diluent extracted. While films normally are stretched after extraction of
the diluent, it is also possible to stretch the films with the diluent
present, in which case porosity may or may not develop.
For small scale evaluation work, a machine such as those made by T. M. Long
Co. (Sommerville, N.J.) can be used. The film is inserted into the machine
at the desired temperature and gripped by all four edges such that the
film can be stretched in one direction (uniaxial) of both directions
(biaxial). Biaxial stretching can be performed sequentially or
simultaneously. For in-line processing, film can be stretched lengthwise
using a device having a series of rollers that can be set to rotate at
increasingly higher rpm. Stretching in the width direction can be
accomplished by a device referred to as a tenter. A tenter can have
several zones that can be heated to a desired temperature. Moving grips
that ride on a rail through the tenter grab the film by the edges. The
spacing between the two sets of grips on either side of the tenter can be
increased as the film moves through the tenter to accomplish the desired
degree of stretching. Available in-line equipment can be simultaneous
biaxial stretching.
In general, bubble point increases and Gurley value decreases as the
stretch ratio (the ratio of final film dimension to initial film
dimension) increases, although an extremum frequently is reached such that
higher stretch ratios result in a lower bubble point and higher Gurley
value. The thickness of the film generally is reduced by stretching. In
the case of conductive carbon particle-filled porous films, stretching has
similar effects on bubble point and Gurley value as with unfilled films
but also tends to increase the resistivity of the film. Careful
optimization is needed to balance suitably the bubble point, Gurley value
and resistivity. In contrast, stretching tends to reduce the resistivity
of porous films loaded with metallic particles such as tungsten.
Unstretched films containing high loadings of tungsten have high
resistivity, which decreases as the stretch ratio is increased.
D. MEA Formation
The catalytic, electrode layer generally is formed as an integral part of
either the ion conducting membrane or the electrode backing layer. In
either case, an electrode backing layer is placed on each side of the ion
conduction membrane with a catalyst layer between each electrode backing
layer and ion conducting membrane to form the 5-layer MEA. The electrode
backing layers and the ion conducting membrane must be held closely
together in order to reduce resistance to ionic and/or electrical flow
between the elements.
The elements can be held together by stack pressure, generally with a
container ultimately applying the pressure. Preferably, the elements are
laminated together. Lamination supplies the physical proximity, as an
alternative to stack pressure. Surprisingly, the lamination step can be
performed with particle-filled, porous polymer components without
destroying the porous characteristic or structural integrity of the
elements.
The lamination step should form cohesive association between the five
layers of the MEA. Selection of appropriate conditions for the lamination
is based on the specific materials used. Particular examples are described
below in the Examples. Lamination conditions should not compromise
membrane properties such as porosity, surface wetting and electrical
resistance.
The objective of the lamination is to eliminate the physical gap between
the layers. Cohesion or self-adhesion of polymers of the different layers
can be promoted by increasing the total area of contact, thus increasing
the probability of diffusional interlacing of polymer chains at the areas
of contact. Some preferred polymer components described above are more
compressible than typical polymer films. Increased compressibility makes
pressure more effective in increasing contact area. Evidently, the
particulate filler in the polymer, electrode backing layer helps to
inhibit the collapse of the pores during the lamination.
Lamination can be accomplished in a variety of ways. These approaches
include the use of heat lamination, pressure lamination or solvent
lamination. Heat lamination and solvent lamination also can involve some
addition of pressure. The appropriate methods for lamination depend on the
materials.
Continuous roll processing of the MEA greatly enhances the efficiency of
fuel cell production. For example, the 5-layer NEA is fabricated as a
continuous web 200 of identical repeating MEAs 202, i.e., as illustrated
in FIG. 3. On the continuous web of MEAs 200, catalyst electrode areas
204, including catalyst layers 206 and electrode backing layers 208, can
be applied patch-wise or continuously on each side to ion conduction
membrane 210, supplied in roll form. Similarly, appropriate seals and
gaskets 212, defined by the mating surfaces of the bi-polar plates, can be
applied at the appropriate locations on roll membrane 210 adjacent
catalyst electrode areas 204. Holes 214 are punched at appropriate
locations at the center of seals or gaskets 216. The boundary between
adjacent MEAs can be indicated for cutting or partially perforated for
fast and easy separation during the stack assembly process. In addition,
registration marks can be applied at the appropriate spots to facilitate
robotic pick-up and alignment during the stack assembly process.
If catalyst layer 206 is associated with electrode backing layer 208, the
combined layers can be attached or laminated to ion conduction membrane
210. Alternatively, catalyst layer 206 and electrode backing layer 208 can
be associated with membrane 210 sequentially. Suitable methods for
attaching or applying catalyst layer 206 to ion conduction membrane 210
depends on the type of catalyst layer 206. For dispersions of carbon
particle supported catalysts, methods such as those taught in U.S. Pat.
No. 5,211,984, incorporated herein by reference, using heat and pressure
can be used. Nanostructured catalyst layers as taught in U.S. Pat. No.
5,338,430 can be embedded in the surface of membrane 210 using nip-roll
calendering or rapid static pressing of a continuous roll supply of the
nanostructured catalyst into a continuous roll supply of membrane 210. The
catalyst can be applied in a patch-wise fashion from a continuous roll
carrier holding the catalyst in the desired pattern.
Electrode backing layers 208 then can be applied in registry with catalyst
electrode area 204 of ion conduction membrane 210 in a patch-wise fashion.
Electrode backing layers 208 and catalyst layer 206 can also be applied in
a continuous roll supply rather than in patch-wise fashion. Various
attachment methods can be used for securing the electrode backing layers
208 prior to stack assembly. Suitable attachment methods for electrode
backing layer 208 include pressure lamination, heated nip-roll lamination,
limited area adhesive attachment (to avoid blocking all pores with
adhesive), ultrasonic welding, microstructured surface mechanical
attachment and the like. A secure bonding of electrode backing layer 208
with membrane 210 generally is desirable to minimize electrical and/or
ionic resistivities across the interface between them, or to facilitate
water management at the interface, especially the cathode interface. The
parameters of the attachment process can be adjusted to provide the
preferred degree of bonding. More secure bonding is especially desirable
when catalyst layer 206 is applied first to electrode backing layer 208.
Important requirements for the attachment methods are that the gas
transport properties of electrode backing layers 208 are not adversely
affected, that catalyst layers 206 are not poisoned and that ion
conduction properties of the membrane 210 are not degraded.
Seals and gaskets 212, 216 can be fabricated or die-cut from any suitable
laminar web material, such as Teflon.TM. sheeting or Teflon.TM. coated
fiberglass sheeting available from The Furon Co., CHR Division (New Haven,
Conn.) or other fluoroelastomers. The seal material can be applied to
perimeter seal points 212 or gas port edges 216 of MEA roll 200.
Attachment of the seals and gaskets to the membrane at those points can be
done using attachment methods similar to those described above for
attaching the electrode backing layers. In addition to the non-adhesive,
laminar web seal materials, appropriate transfer adhesives also can be
used. An example of such a transfer adhesive is #9485 PC adhesive
available from 3M Co. (Saint Paul, Minn.).
The seals and gaskets materials and corresponding adhesives should not
contain chemicals that can be extracted by the ion conduction membrane to
lower its conductivity or poison the catalyst. Also, the seals and gaskets
materials should be chemically and thermally inert to withstand the acidic
environment (for proton exchange fuel cells) and operating temperatures of
the fuel cell for thousands of hours. Furthermore, seals and gaskets 112,
116 should have adequate mechanical properties to have high resistance to
creep and extrusion at the maximum operating temperatures of the stack
under stack-applied compressive forces in the direction normal to the seal
areas and under forces acting in the plane of the seals generated by
internal pressure.
E. Stack Formation
A typical fuel cell stack may require more than a hundred cells to be
assembled in series to obtain useful voltages. A hundred cells in series,
each operating at a nominal 0.7 volts, would yield a 70 volt stack.
Assembly of the MEAs and bi-polar/cooling plates with all the attendant
gaskets and seals to produce a leak free, optimally compressed fuel cell
stack can be a critical issue for reducing the cost of the stack.
Providing the MEAS, seals and gaskets in a manufacture-ready format to
facilitate cost effective assembly of a stack is an important issue. For
example, to assemble only 10,000 fuel cell stacks per shift per production
line per year requires one stack with hundreds of associated cell
components to be assembled approximately every 10 minutes. Producing and
handling such a large number of components, each sized, cut, oriented and
held in proper registry, in such a short time is a significant
consideration.
In a fuel cell stack 300, each individual cell consists of a 5-layer MEA
302 that is sandwiched between bi-polar plates 304, as shown in FIG. 4.
End plates 306 provide for flow of fuel and oxidizing agent into and out
from fuel cell stack 300. The bi-polar plates function to a) provide the
series connection between cells by conducting the total electrical current
produced by an MEA to the adjacent cell for eventual transmission at the
end plates, b) prevent any gas transport between adjacent cells, c)
provide mechanical rigidity to the assembled stack such that compressive
forces are effective to minimize leakage of gases past the perimeter of
the MEAs, d) provide flow field grooves and gas manifold ports for
introducing the fuel and oxidant to the MEA catalyst electrodes and for
removal of by-products such as water and e) provide contact with cooling
fluids to extract waste heat from the cell electrode areas.
Although there can be many possible configurations and shapes for fuel cell
stacks, generally they are rectilinear or cylindrical in shape so that the
individual planar MEAs and bi-polar plates within each cell have a
corresponding rectangular or circular shape. U.S. Pat. No. 5,252,410,
incorporated herein by reference, teaches many aspects of bi-polar plates
and stack assemblies including specific aspects for the case in which the
catalyst is applied to the electrode backing layer. The active, catalyzed
area of each MEA generally is smaller than the membrane area and can be
centered on the MEA. The perimeter area of the membrane bordering the
electrode area generally is used for sealing the MEA to the bi-polar
plates, to prevent leakage of fuel and oxidant from the pressurized
interior of the cell. Compressive forces applied from the end plates of
the stack should be sufficient to keep the gaskets or seals from
delaminating at the maximum internal pressures. The region of the MEA
adjacent to the electrode area may also contain holes for transmission of
fuel and oxidant to the cells from the respective gas supply manifolds.
These holes (i.e., gas ports) also may require seals or gaskets to prevent
leakage.
Above, a process is described for fabricating MEA's and supplying the MEAs
with appropriate seals and gaskets in a continuous web format. The
continuous web format is extremely well suited for producing and handling
the large number of MEA elements used to construct the fuel cells at a
cost effective rate. The continuous web is not only well suited for
relatively rapid application to a bi-polar plate but also for accurate
alignment of the MEA. Therefore, the electrode backing layers as described
herein when adapted to the production of a 5-layer MEA in a continuous
roll format yield dramatic advances in fuel cell processing.
EXAMPLES
Several properties are measured for the various electrode backing layers
produced in the following examples. Bubble point is the largest pore size
in the film as determined according to ASTM F-316-80. Ethanol was used as
the test liquid. The liquid is used to fill the pores of the film.
Pressure is applied until flow as bubbles takes place through the largest
passageway through the film. The bubbles are observed from a tube that is
connected to the low pressure side of the test cell and that is submerged
in water. The necessary pressure depends on the surface tension of the
test liquid and the size of the largest passageway. Bubble point in
microns, using ethanol as the test liquid, is equal to 9.25/pressure in
psi at breakthrough.
Gurley value is a measure of resistance of air flow through a film.
Specifically, it is a measurement of the time in seconds for 100 cc (or
other selected volume) of air to pass through one square inch of a film at
a pressure of 124 mm of water, according to ASTM D-726-58, Method A. The
film sample is clamped between two plates. Then, a cylinder is released
that provides air to the sample at the specified pressure. The time for a
given amount of air flow is determined from the marks on the cylinder,
which are read electronically. In the Examples, Gurley values are reported
for passage of 50 cc or 10 cc of air.
The in-plane electrical resistance is measured using two, 1.5 cm wide
aluminum bars that are placed parallel to each other on the surface of the
film. Weights were placed on top of the bars to give a pressure of 300
g/cm.sup.2. The results generally are pressure dependent. The resistance
between the two aluminum bars was measured using a standard ohm meter.
Alternatively, z-axis electric resistance was measured at high current
densities, as described in Example 6, below. The resistivity in ohm-cm was
calculated using the following equation:
resistivity=(z-axis resistivity.times.area of film/thickness of film)
or
resistivity=(in-plane resistance.times.width of the film.times.film
thickness)/distance between bars
The aqueous contact angle measurements described below were performed
essentially as described in WO 96/344697. Briefly, using a commercial
apparatus (Rame-Hart contact Angle Goniometer, Model 100), an
approximately 1 microliter droplet was expressed out a hypodermic syringe.
Carefully raising the sample surface to just contact the droplet while
still suspended from the syringe defined the "equilibrium contact angle".
The droplet was then enlarged or shrunk while measuring the contact angle
to obtain the advancing and receding contact angles, respectively.
Multiple measurements were made and the mean and rms deviation obtained
for both types of contact angles at multiple points on the surface.
Membranes exhibiting a higher receding contact angle repel water to a
greater extent that those exhibiting a lower receding contact angle.
Without wishing to be bound by theory, it is believed that membranes that
repel water to a greater extent are less likely to be flooded during the
operation of a fuel cell, and will allow better flow of fuel and oxidant
to the membrane/catalyst interface.
In the Examples:
"room temperature" or "ambient temperature" is taken as approximately
22.degree. C.;
Vertrel 423.TM. is dichlorotrifluoroethane (CHCl.sub.2 CF.sub.3), from
DuPont Chemicals, Inc., Wilmington, Del.; and
All other chemicals and reagents were obtained from Aldrich Chemical Co.,
Milwaukee, Wis., unless otherwise specified.
There are a number of basic processes and materials in common within the
examples. These include the preparation of the nanostructured catalyst
support, application of the catalyst to the support, determination of the
catalyst loading, fabrication of the membrane-electrode assembly, the type
of fuel cell apparatus and testing station, the fuel cell test parameters,
and the kinds of proton exchange membranes used. These are defined in
general as follows:
a) Nanostructured catalyst support preparation and catalyst deposition. In
the following examples, the nanostructured catalyst electrodes and the
process for making them are as described U.S. Pat. No. 5,338,430 and other
patents referenced therein. The nanostructured catalyst consists of
catalyst materials, e.g. Pt coated onto the outer surface of nanometer
sized whisker-like supports. The whiskers are produced by vacuum annealing
thin films (100-1500 Angstroms) of an organic pigment material (C.I.
Pigment Red 149, or PR149) previously vacuum coated onto substrates such
as polyimide. The whisker-like supports, with lengths of 1-2 micrometers,
grow with uniform cross-sectional dimensions of 30-60 nanometers,
end-oriented on a substrate to form a dense film of closely spaced
supports (30-40 per square micrometer) which can be transferred to the
surface of a polymer electrolyte to form the catalyst electrode. The
nanostructured catalyst electrode has a very high surface area which is
readily accessible to fuel and oxidant gases.
b) Measurement of the catalyst loading is done both by monitoring the
thickness of the Pt layer deposited during vacuum coating using a quartz
crystal oscillator, as is well known in the art of vacuum coating, and by
a simple gravimetric method. In the later case, a sample of the polyimide
supported nanostructured film layer is massed using a digital balance
accurate to 1 microgram, and its area measured. Then the nanostructured
layer is wiped off the polyimide substrate using a paper tissue or linen
cloth, and the substrate is remassed. Because a preferred property of the
catalyst support is that it transfer easily and completely to the ion
exchange membrane, it also can be easily removed by simple wiping with a
cloth. The mass per unit area of the catalyst support particles, without
Pt, can also be measured this way.
c) The ion exchange membranes used were all of the perfluorinated sulfonic
acid type. Nafion.TM. 117 or 115 membranes were obtained from DuPont
Corp., Wilmington, Del.
d) The process used for transferring the catalyst coated support particles
into the surface of the PEM or DCC was a static pressing or a continuous
nip-rolling method. To prepare an MEA with e.g. the catalyst on the PEM,
with 5 cm.sup.2 of active area by the static pressing method, two 5
cm.sup.2 square pieces of the nanostructured catalyst, coated on a
metallized polyimide substrate, one for the anode, one for the cathode,
are placed on either side of the center of a 7.6cm.times.7.6 cm proton
exchange membrane. At least one 25 or 50 micrometer thick sheet of
polyimide, of the same size as the PEM, is placed on each side of the PEM
and nanostructured substrate stock to form a stack. For static pressing,
one sheet of 50 micrometer thick Teflon.TM., of the same size as the PEM,
is placed on each side of the PEM, nanostructured substrate and polyimide
stack.
For static pressing, this assembly is then placed between two steel shim
plates, and pressed at a temperature near 130.degree. C. and pressures
approaching 10 tons/cm.sup.2 for up to two minutes, using a nine inch
Carver.TM. press. A low grade vacuum may be applied to partially remove
air (2 Torr) from the stack just prior to applying the maximum pressure.
Before releasing stack pressure, the stack can be cooled usually for 5
minutes or less to near room temperature. The original 5 cm.sup.2
polyimide substrates are then peeled away from the PEM leaving the
catalyst attached to the surface of the PEM. (Alternatively the catalyst
support particles can be transferred to the PEM or electrode backing by
continuous toll processes such as passing the above sandwich assemblies in
continuous or semi-continuous sheet form through the nip of a mil as in
calendering or laminating processes. The two mill rolls can be heated,
both made of steel, or steel and a softer material such as rubber, have a
controlled gap or use controlled line pressure (kg/cm) to determine the
gap of the nip.
e) The MEA's from step d) were mounted in a fuel cell test cell purchased
from Fuel Cell Technologies, Inc., Albuquerque, N. Mex., generally a 5
cm.sup.2, but up to 50 cm.sup.2, sized cell. Two pieces of 0.015" thick
ELAT electrode backing material, obtained from E-tek, Inc., Natick, Mass.
was used as control electrode backing material. Teflon coated fiberglass
gaskets, purchased from CHR Industries, nominally 250 micrometers thick,
with 10 cm.sup.2 square holes cut in the center (for the 10 cm.sup.2
catalyst area), were used to seal the cell. The ELAT electrode backing
material is designated as carbon only, i.e. it contains no catalyst.
f) The test parameters for the fuel cell polarization curves of examples
9-14 and 28, were obtained under the conditions of 207 kPa H.sub.2 and 414
kPa oxygen gauge pressures with a cell temperature of 80.degree. C., flow
rates of approximately one standard liter per minute. The humidification
of the gas streams was provided by passing the gas through sparge bottles
maintained at 115.degree. C. and 80.degree. C. respectively for the
hydrogen and oxygen.
For examples 15-17, polarization curves were obtained to test the low
pressure air performance of the electrode backing materials. The curves in
FIG. 12 were obtained under the conditions of 207 kPa H.sub.2 and 34.5 kPa
air gauge pressures. The H.sub.2 /air flow rates were 400/400 sccm
(standard cubic centimeters per minute) for 10 cm.sup.2 MEAs, and 1
standard liters per minute (slm)/2 slm for the 50 cm.sup.2 MEAs. The
humidification of the gas streams was provided by passing the gas through
sparge bottles maintained at about 115.degree. C. and 65.degree. C.,
respectively, for the hydrogen and air. The cell temperature was
75.degree. C. A membrane produced following the procedures described in
Example 12 was also run under these fuel cell conditions. The results are
plotted in FIG. 12 as Ex. 12 (air).
Example 1
Conductive Carbon in High Density Polyethylene
A dispersion of conductive carbon in mineral oil was prepared by wetting
out 1032 g of Conductex.TM. 975 conductive carbon (Colombian Chemicals
Co., Atlanta, Ga.) into a mixture of 2054 g of mineral oil (Superla.RTM.
White Mineral Oil No. 31, AMOCO, Chicago, Ill.) and 1032 g of dispersant,
OLOA 1200.TM. (Chevron Oil Co., San Francisco, Calif.) using a model 2500
HV dispersator Premier Mill Corp., Reading, Pa.). Portions of the carbon
and OLOA 1200 were added alternately to the mineral oil. As the carbon was
added, the viscosity increased and the dispersator rpm increased
accordingly to a maximum of about 5000 rpm after all the carbon and OLOA
1200 had been added.
The resultant dispersion was viscous and lumpy. It was then heated to about
150.degree. C. while continuing to mix with the dispersator to degas it.
The viscosity decreased as the temperature increased; the dispersator rate
was reduced to 1100 rpm as the temperature increased. The mixture was held
at about 150.degree. C. for 20 min. The dispersion became smoother with
continued mixing and heating. It was then allowed to cool to about
60.degree. C. while continuing to mix. The resulting mixture was passed
through a 1.5 L horizontal mill (Premier Mill Corp.) containing an 80 vol.
% charge of 1.3 mm diameter chrome-steel beads. The horizontal mill was
operated at a peripheral speed of 1800 fpm (54.9 meters/minute) and at a
through put rate of about 0.5 L/min.
The dispersion discharged from the horizontal mill was pumped at about
60.degree. C. into an injection port on the third zone of a Berstorff.TM.
co-rotating twin screw extruder (25 mm.times.825 mm, Berstorff Corp.,
Charlotte, N.C.). HDPE (high density polyethylene, grade 1285, Fina Oil &
Chemical Co., Houston, Tex.) was metered into the feed zone (zone 1) at a
rate of 0.55 kg(1.20 lb.)/hr. and the above dispersion was pumped in at a
nominal rate of 69.1 cc/min. using a gear pump. The extruder profile
starting from the feed zone was 193, 254, 254, 204, 166, 160, 166.degree.
C., the die temperature was 166.degree. C., and the screw speed was 120
rpm.
Film was extruded through an 20.32 cm (8 in.) die onto a patterned casting
wheel heated to 52.degree. C. The wheel pattern had 45.degree., four-sided
pyramids that were 0.125 mm (5 mil) high at a density of 100 per 6.45 sq.
cm ( 1 square inch). The resultant film was 0.25 mm (10 mil) thick and the
experimentally determined total film throughput rate was 4.53 kg (10.0
lb)/hr. Thus, the actual dispersion feed rate was 3.99 kg (8.8 lb)/hr.
From this and the known dispersion composition, the total carbon content
in the film after extraction of the oil was calculated to be 64.7 wt. %.
Example 2
Extraction using Vertrel 423.TM.
Oil and OLOA 1200 were extracted from the film of Example 1 in three washes
by soaking a portion of the film measuring about 18 cm by 30 cm in about
1L Vertrel 423.TM. solvent per wash for 10 minutes per wash. On drying at
room temperature, film thickness was 0.0241 cm. Physical properties of the
film are shown in Table 1.
Example 3
Extraction using Toluene/Xylenes
Oil and OLOA 1200 were extracted from a portion of the film of Example 1 as
described in Example 2, using a 1:1 v/v mixture of toluene/xylenes.
Physical properties of the dried film, measuring 0.023 cm thick, are shown
in Table 1.
Example 4
Post-extraction Heating, Vertrel 423.TM. extraction
A portion of the film prepared as described in Example 2 was hung in a
circulating air oven for ten minutes at 130.degree. C. On cooling, the
film measured 0.23 mm thick. Physical properties of the film, labeled
"Example 4A", are shown in Table 1. Advancing and receding contact angles
(water) for the film were 158.degree. and 107.degree., respectively.
Likewise, a portion of the film from Example 2 was heated in a circulating
air oven for 10 minutes at 150.degree. C. Physical properties of the film,
labeled "Example 4B", are shown in Table 1.
Example 5
Post-extraction heating, Toluene/Xylenes extraction
Films prepared as described in Example 3 was hung in a circulating air oven
for ten minutes at 130.degree. C. On cooling, the film measured 0.23 mm
thick. Physical properties of the film, labeled "Example 5A", are shown in
Table 1.
Likewise, a portion of the film from Example 3 was heated in a circulating
air oven for 10 minutes at 150.degree. C. Physical properties of the film,
labeled "Example 5B", are shown in Table 1.
TABLE 1
X-Y
Extraction Heated, Bubble Gurley no., resistivity*
Example Solvent .degree. C. Point, .mu.m sec/50 cc ohm-cm
2 V.sup.1 No 0.10 310 6.7
3 T/X.sup.2 No -- -- 0.97
4A V 130 0.15 180 0.75
4B V 150 -- -- 0.67
5A T/X 130 0.14 105 0.53
5B T/X 150 -- -- 0.53
.sup.1 Vertrel 423 .TM.
.sup.2 Toluene/Xylenes (1:1 v/v)
*low current measurement between parallel aluminum bars with 0.5
kg/cm.sup.2 applied pressure
As shown in Table 1, heating the film above the melting point of the HDPE
binder (126.degree. C., peak temperature by DSC) resulted in a significant
decrease in Gurley, a significant increase in bubble point, and a
significant decrease in resistivity.
Example 6
Film Impedance
The ability of carbon-loaded films of the invention to carry large current
densities, suitable for fuel cells, was demonstrated. Two 5 cm.sup.2
square samples of the film from Example 4A (Vertrel 423.TM. extraction,
130.degree. C. heating) were mounted face-to-face in direct parallel
contact with one another in a fuel cell test fixture (2.24 cm.times.2.24
cm, Fuel Cell Technologies, Inc., Santa Fe, N. Mex.), i.e., there was no
intervening ion conductive membrane between the samples. Masking frames of
Teflon.TM. impregnated fiberglass 0.015 cm thick were used between the
test cell halves as commonly used in actual fuel cell tests, to prevent
crushing the films to be examined. Cell bolts were torqued to 12.4 N-m
(110 in-lbs). High current levels were passed through the cell at various
voltages to measure the impedance of the films under high current density
conditions. Results of these measurements are shown in FIG. 6, trace A.
After measurements were taken, the combined thickness of the films was
0.042 cm. Resistivity of the films was measured as 0.57 ohm-cm, comparable
to the value shown in Table 1.
Example 7
Film Impedance
Films prepared in Example 5A (toluenelxylenes extraction, 130.degree. C.
heating) were examined as described in Example 6 to measure their
impedance. Results are shown in FIG. 6, trace B. Measured resistivity of
these films, having a combined thickness of 0.042 cm, was 0.52 ohm-cm.
Example 8 (Comparative)
Film Impedance
The resistivity of a carbon-only material (woven graphite cloth
impregnated/coated with carbon black/PTFE) commercially available as
ELAT.TM. (Etek, Inc., Natick, Mass.) was examined as described in Example
6. Results are shown in FIG. 6, trace C. Combined thickness of the
ELAT.TM. films was 0.094 cm, giving an effective bulk resistivity of 0.28
ohm-cm. The Gurley value of the ELAT.TM. material was measured to be 7.5
sec/50 cc, and the advancing and receding contact angles of the film were
155.degree. and 133.degree., respectively.
Example 9
Membrane Electrode Assembly
A proton exchange membrane electrode assembly (MEA) was prepared by
applying an electrode layer comprising platinum-coated nanostructured
supports, as described in U.S. Pat. No. 5,338,430, the teachings of which
are incorporated herein by reference, to the central portion of a 7.6
cm.times.7.6 cm square Nafion.TM. 117 ion exchange membrane (DuPont
Chemicals Co., Wilmington, Del.). The platinum-coated nanostructured
supports were applied to both sides of the ion exchange membrane using a
hot platen press as described in Example 5 of the above-incorporated '430
patent. The centered electrode area was 5 cm.sup.2. Two 5 cm.sup.2 pieces
of the carbon-filled electrode backing layer formed as described above in
Example 5A (toluene/xylenes extraction, 130.degree. C. heating) were
placed on either side of the electrode assembly, to form a 5-layer MEA.
The assembly was mounted in a 5 cm.sup.2 test cell and tested on a fuel
cell test station (Fuel Cell Technologies, Inc.), using hydrogen/oxygen
gas flows applied to respective sides of the assembly. FIG. 7, trace A,
shows a polarization curve of voltage vs. current density produced with
this assembly.
Example 10
Membrane Electrode Assembly
A membrane electrode assembly was prepared as described in Example 9 except
that an electrode backing layer as described in Example 3 (toluene/xylene
extraction, no heating) was used. In addition, the entire assembly
comprised 50 cm.sup.2 electrodes and electrode backing membranes, rather
than 5 cm.sup.2. FIG. 7, trace C, shows a polarization curve of voltage
vs. current density produced by this assembly. The improved performance of
this cell can be attributed, in part, to the larger electrode size.
Example 11 (Comparative)
Membrane Electrode Assembly
A membrane electrode assembly was prepared as described in Example 9 except
that the ELAT.TM. material described in Example 8 was used as the
electrode backing layer. FIG. 7, trace B, shows a polarization curve of
voltage vs. current density produced by this assembly.
Examples 9-11 show that an effective electrode backing layer of the
invention can be prepared by the TIPT method. A premium grade
commercially-available membrane provided better fuel cell performance,
perhaps due, in part, to a lower Gurley value and a higher receding
contact angle. A lower Gurley value and a higher receding contact angle
may be indicative of higher diffusion of hydrogen and oxygen to the
catalyst/electrolyte interface and lesser susceptibility to flooding with
water produced at the cathode, which would further limit oxygen transport.
Example 12
Graphite/Conductive Carbon (95/5) in Ultrahigh Molecular Weight
Polyethylene (TIPT)
A dry blend of 37.11 g MCMB 6-28 graphite (nominally 6 .mu. mean diameter,
Osaka Gas Chemical Co., Osaka, Japan) and 1.91 g Super P conductive carbon
(MMM Carbon Div:, MMM nv, Brussels, Belgium) was prepared using a spatula
for mixing. Portions of this mixture and portions of 32.2 g mineral oil
(Superla.RTM. White Mineral Oil No. 31) were added alternately to the
mixing chamber of a Haake Rheocord.TM. System 9000 (Haake (USA), Paramus,
N.J.) equipped with roller blades. The mixing chamber was at 60.degree.
C., while mixing at 50 rpm. Then, heating to a set point of 150.degree. C.
was begun.
When the mix temperature reached 120.degree. C., 2.06 g of ultrahigh
molecular weight polyethylene (UHMWPE, grade GUR 4132, Hoechst Celanese
Corp., Houston, Tex.) was added in portions with time allowed between
additions for the previous material to be assimilated. The ratio of
UHMWPE/oil was 6/94. After this addition was completed, the temperature of
the chamber was increased to 150.degree. C., and the mixing rate was
increased to 80 rpm. Mixing was continued for 10 min. after the addition
of the UHMWPE had been completed. The mixture was removed from the mixer
while still hot.
After cooling, 15 g of solidified mixture was placed between 0.175 mm (7
mil) polyester sheets and placed in a Model 2518 Carver.TM. press (Fred S.
Carver Co., Wabash, Ind.) at 160.degree. C. with 0.25 mm (10 mil) shims
placed between the polyester sheets. After heating in the press for 3 min.
with no applied pressure, the mixture was pressed for 10 sec. using 690
kPa (100 psi). The resultant film with polyester sheets still attached was
immersed into water at ambient temperature to quench it. The oil was
extracted from the film as described in Example 2. A portion of the film
was heated at 130.degree. C. for 10 min. in a circulating air oven, as
described in Example 4. Physical properties of the film are shown in Table
2. Advancing and receding contact angles (water) for the film were
154.degree..+-.10 and 101.degree..+-.5, respectively.
TABLE 2
After Washing/
Drying, Before After Heating for 10
Parameter Heating min. at 130.degree. C.
Caliper, mm 0.215 0.215
Gurley (sec./50 cc) 95 60
Bubble Point (microns) 0.23 --
Resistivity (ohm-cm) 7.3 4.6
Curve A in FIG. 8 shows a representative polarization curve from a fuel
cell test using a 50 cm.sup.2 cathode backing layer made after heating, as
described in this example. The same catalyst coated ion conduction
membrane was used to obtain the fuel cell polarization curves for the
5-layer MEAs using different electrode backing layers of Examples 12-14
and the ELAT.TM. control (curve "D" in FIG. 8). After testing one sample
electrode backing layer, the test cell was opened, and the electrode
backing layer replaced on the cathode with the next one. The ELAT.TM.
anode backing layer remained unchanged.
Example 13
Graphite/Conductive Carbon (95/5) in Polypropylene (TIPT)
A mixture of MCMB 6-28 graphite and mineral oil (Superla.RTM. White Mineral
Oil No. 31) was prepared by mixing 83.3 g of graphite into 91.9 g of
mineral oil using a dispersator. Super P conductive carbon, 1.53 g, was
poured into the mixing chamber of a Haake Rheocord.TM. System 9000 mixer
equipped with roller blades at 100.degree. C. Then, while mixing at 50
rpm, 59.73 g of the graphite/mineral oil mixture was poured into the
mixing chamber. As the viscosity increased during the addition of the
graphite/mineral oil mixture, the mixing rate was increased to 100 rpm.
Then, 7.66 g of polypropylene (grade DS5D45 from Shell Chemicals, Houston,
Tex.) were added. The mixture was heated to 230.degree. C. over a period
of about 10 min. Total mixing time after addition of polypropylene was
about 33 min. The resultant mixture was removed from the mixer while hot.
After cooling, 14.2 g of the solidified mixture was placed between 0.175 mm
(7 mil) polyester sheets, which had been coated with a thin coating of
mineral oil to facilitate release, and placed in a Carver press at
160.degree. C. with 0.25 mm (10 mil) shims placed between the polyester
sheets. After heating in the press for 3 min. with no applied pressure,
the mixture was pressed for 10 sec. using 345 kPa (50 psi). The resultant
film with polyester sheets still attached was immersed in water at ambient
temperature to quench it. Oil was extracted from the film as described in
Example 2. A portion of the film was heated in a circulating air oven for
10 min. at 180.degree. C. Physical properties of the film before and after
heating are shown in Table 3. It was noted that the film became somewhat
brittle after this heating procedure. Advancing and receding contact
angles (water) for the film were 155.degree..+-.5 and 100.+-.5.degree.,
respectively.
TABLE 3
After Washing/
Drying, Before After Heating for 10
Parameter Heating min. at 180.degree. C.
Caliper (mm) 0.205 0.200
Gurley (sec./50 cc) 32 --
Bubble Point (microns) 0.66 --
Resistivity (ohm-cm) 8.96 1.5
Trace B in FIG. 8 shows a representative polarization curve with the
cathode electrode backing layer made from the heated film. It displays an
improved performance relative to the sample from Example 12.
Example 14
Graphite/Conductive Carbon (95/5) in Ultrahigh Molecular Weight
Polyethylene (TIPT)
A dry blend of 27.89 g MCMB 6-28 graphite, and 1.47 g Super P conductive
carbon, was prepared using a spatula for mixing. Portions of this mixture
and portions of 37.1 g mineral oil (Superla.TM. White Mineral Oil No. 31)
were added alternately to the mixing chamber of a Haake Rheocord.TM.
System 9000 mixer equipped with roller blades at 40.degree. C. while
mixing at 50 rpm. Then, 1.55 g of UHMWPE (grade GUR 4132, Hoechst Celanese
Corp.) were added. The ratio of UHMWPE/oil was 4/96. After addition of the
polymer was completed, the temperature of the chamber was increased to
150.degree. C., and the rpm were increased to 80. Mixing was continued for
10 min. after the addition of the UHMWPE had been completed. The mixture
was removed from the mixer while still hot.
After cooling, 13.1 g of the solidified mixture was placed between 0.175 mm
(7 mil) polyester sheets and placed in a Carver press at 160.degree. C.
with 10 mil shims placed between the polyester sheets. After heating in
the press for 3 min. with no applied pressure, the mixture was pressed for
10 sec. using 345 kPa (50 psi). The resultant film with polyester sheets
still attached was immersed into water at ambient temperature to quench
it. The oil was extracted from the film as described in Example 2. A
portion of the film was heated at 130.degree. C. for 10 min. in a
circulating air oven, as described in Example 4. The peak melting point of
the UHMWPE was 138.degree. C. as determined by DSC. Physical properties of
the film are shown in Table 4. Advancing and receding contact angles
(water) for the film were 139.degree..+-.10 and 79.degree..+-.9,
respectively.
TABLE 4
After Washing/
Drying, Before After Heating for 10
Parameter Heating min. at 130.degree. C.
Caliper (mm) 0.150 0.150
Gurley (sec./50 cc) 36.8 19.6
Bubble Point (microns) 0.93 0.60
Resistivity (ohm-cm) 36 9.7
Trace C in FIG. 8 displays a representative polarization curve with a
cathode electrode backing layer made from the heated film. Further
improvement is observed relative to Example 13. Trace D involves the
ELAT.TM. control.
In the following Examples 15A, 15B, 16A, 16B, 17B, 17C, 17D, 17E and 17F,
equivalent catalyst coated ion conduction membranes were used for tests of
different types of cathode backing layers. Commercial ELAT.TM. was used in
each case as the anode backing layer. The fuel cell polarization curves
for these examples are summarized in FIG. 12, and demonstrate the effects
of the different parameters tested under low pressure air operation. The
comparative control curve with ELAT.TM. as the cathode backing layer is
also shown in FIG. 12. Referring to FIG. 12, in the useful voltage range
of 0.6 volts and higher, the electrode backing layer of Example 15A
exceeds the performance of the ELAT.TM. membrane.
Example 15
Graphite/Conductive Carbon in Polyvinylidene Fluoride
A mixture of 91.37 g of MCMB 6-28 graphite in 96.18 g of propylene
carbonate was prepared by using a dispersator. Then, 1.60 g Super P
conductive carbon was added to the mixing chamber of a Haake Rheocord.TM.
System 9000 mixer at 50.degree. C. and 50 rpm, followed by addition of
63.0 g of the above graphite-propylene carbonate mixture. While heating
the resulting mixture to 150.degree. C., 12.47 g Solef 1010.TM.
polyvinylidene fluoride (PVDF, Solvay America Inc., Houston, Tex.) was
added in portions at a rate such that the added polymer was assimilated
into the mixture. When steady torque had been established (approximately 6
minutes after commencing polymer addition), the temperature set point was
changed to 120.degree. C. and cooling commenced. After approximately 4
minutes of cooling, stirring was stopped and the resulting mixture removed
while hot.
After cooling, 12 g of solidified mixture was placed between two sheets of
polyimide film with 0.25 mm (10 mil) shims between the polyimide film, and
placed in a Carver press at 150.degree. C. After heating for 90 sec. with
no applied pressure, the press was closed for 5 seconds using 1035 kPa
(150 psi). The resultant film with polyimide sheets still attached was
placed between two 15 mm thick steel plates at 20.degree. C. until the
film was cool, after which the polyimide film was removed. The resultant
PVDF film was washed and then dried as described in Example 2, except that
3.times.1 L isopropyl alcohol washes were used to extract the propylene
carbonate to give sample 15A. A portion of the film was heated at
160.degree. C. for 10 min. in a circulating air oven, as described in
Example 4 to give sample 15B. Physical properties of the film are shown in
Table 5. The fuel cell polarization results are shown in FIG. 12.
TABLE 5
After Washing/
Drying, Before After Heating for 10
Heating (15A) min. at 160.degree. C. (15B)
Caliper, mm 0.241 0.230
Gurley (sec./50 cc) 57 39
Resistivity (ohm-cm) 1.20 0.96
Advancing Contact Angle 143 .+-. 10.degree. 141 .+-. 8.degree.
Receeding Contact Angle 87 .+-. 12.degree. 87 .+-. 8.degree.
Example 16
Graphite/Super S Conductive Carbon (95/5) in High Density Polyethylene
(TIPT)
This example demonstrates useful performance at a much lower loading of
carbon. This film was made using the extruder described in Example 1 and
was cast onto a smooth casting wheel (32.degree. C. set point
temperature). Film made this way has typically smaller pores on the wheel
side than on the air side.
A dispersion of SFG 15 graphite (Alusuisse Lonza America Inc., now Timcal,
Fair Lawn, N.J.) was prepared by adding incrementally 1090 g of SFG 15 to
a mixture of 3030 g of mineral oil (Superla.TM. White Mineral Oil No. 31)
and 57.4 g of dispersant, OLOA 1200 using a Model 89 dispersator from
Premier Mill Corp. Then, 57.4 g of Super S conductive carbon MMM Carbon
Div., MMM nv, Brussels, Belgium) was mixed into the graphite dispersion.
The carbon/oil mixture was heated to 150.degree. C. and held at
150.degree. C. for 30 min. while continuing to mix with the dispersator
(rpm were lowered as temperature increased). The mixture was cooled to
70.degree. C. before being transferred to the feed tank of the extruder.
The carbon/oil mixture was pumped into an injection port on the third zone
of a Berstorff.TM. co-rotating twin screw extruder (25 mm.times.825 mm).
High density polyethylene (HDPE, grade 1285, Fina Oil & Chemical Co.) was
metered into the feed zone (zone 1) at a rate of 0.61 kg(1.35 lb)/hr., and
the above mixture was pumped in at a nominal rate of 77 cc/min. using a
gear pump. The extruder profile starting from the feed zone was 199, 271,
271, 188, 188, 188, 188.degree. C., the die temperature was 188.degree.
C., and the screw speed was 125 rpm.
Film was extruded through an 20.32 cm (8 in.) die onto a smooth casting
wheel at 32.degree. C. A 50 micrometer polyester film was inserted on top
of the film after quenching while still on the casting wheel to aid in
film handling by preventing slippage on the wheel. The resultant extruded
film was 0.3 mm (12 mil) thick and the experimentally determined total
film throughput rate was 5.39 kg (11.9 lb.)/hr. Thus, the actual
carbon/oil mixture feed rate was 4.80 kg (10.6 lb.)/hr. From this and the
known compositions, the total carbon content in the film after extraction
of the oil was calculated to be 68.0wt. %
The oil and OLOA 1200 were extracted from the film using three.times.15
min. washes using Vertrel 423. About 1 L of solvent per wash was used for
a piece of film that was about 17.8 cm (7") wide by 30.5 cm (12") long.
The film was then hung in an exhaust hood to dry to give sample 16A. A
piece of this film was hung in a circulating air oven for 10 min. at
130.degree. C. to give sample 16B. As shown in Table 6, below, heating the
film above the melting point of the HDPE used, 126.degree. C., resulted in
a significant decrease in Gurley value, significant increase in bubble
point, and significant decrease in resistivity. Physical properties of the
film are shown in Table 6. Fuel cell polarization curves using these
membranes are shown in FIG. 12. For both 16A and 16B, the casting wheel
side of the film was toward the MEA. The photomicrographs of the casting
wheel side, air side and cross sections of films 16A and 16B are shown in
FIGS. 14 and 15, respectively. The SEM results show the differences in
pore size between the casting wheel and air sides of the films, and the
general enlargement of pore sizes throughout film 16B due to heating, as
described.
TABLE 6
After Washing/
Drying, Before After Heating for 10
Heating (16A) min. at 130.degree. C. (16B)
Caliper, mm 0.285 0.274
Gurley (sec./50 cc) 245 20.8
Bubble Point (microns) 0.38 1.16
Resistivity (ohm-cm) 71 1.45
Results given in Examples 17 and 18 below show that similar physical
properties were obtained
1. by either heating the film above the melting point of the HDPE and then
stretching at a normal stretch temperature for porous HDPE (usually about
180 to 220.degree. F.), or
2. by stretching at a higher temperature that would normally result in loss
of porosity of an unfilled HDPE in the membrane.
Example 17
Effect of Stretching and Heating on TIPT Membranes
Samples of the film prepared as described in Example 2 were variously
heated and stretched as shown in Table 7. The film was stretched using a
film stretcher from T. M. Long Co., Somerville, N.J. After inserting the
film into the stretcher at the indicated temperature, the film was heated
for about 30 sec. before stretching. Stretching was performed in one
direction or sequentially in both directions at about 2.54 cm/sec. After
stretching, the films were annealed at the stretching temperature for
about 2 min. before releasing the stretcher grips and removing the
stretched film. In the Table, the degree of stretching is indicated in
terms of the ratio of final dimension divided by initial dimension: a
stretch ratio of 1.25.times.1 means that the film was stretched uniaxially
by 25% (12.7 cm final length, 10.2 cm initial length). 1.25.times.1.25
means that the film was stretched by 25% in both directions, sequentially.
Simultaneous biaxial stretching in both directions is also possible.
TABLE 7
Stretch Bubble Gurley, Resis-
Treat- Stretch Temp., Caliper, Point, sec./ tivity,
Ex. ment Ratio .degree. C. mm .mu.m 50 cc ohm-cm
17A None -- -- 0.216 0.095 426 5.5
17B Heat -- -- 0.225 0.13 210 1.0
Only
17C Heat, 1.25 .times. 87 0.218 0.19 116 1.3
then 1
Stretch
17D Stretch 1.25 .times. 134 0.175 0.40 38 1.1
Only 1
17E Stretch 1.25 .times. 134 0.165 0.47 49 4.2
Only 1.25
17F Stretch 1.5 .times. 134 0.18 0.41 47 2.4
Only 1
In the Table, Example 17A corresponds to a film as prepared in Example 2,
Example 17 B corresponds to a film as prepared in Example 3. For Example
17C., the film from Example 17B was cooled from 130.degree. C. to room
temperature prior to stretching, and then heated in the T. M. Long Co.
stretcher to 93.degree. C. before stretching. For Examples 17D, 17E, and
17F, a film prepared as in Example 2 was heated to the temperature shown
in Table 7 in the T. M. Long Co. film stretcher and then stretched,
without first heating to 130.degree. C., as in the heat-only method.
In general, stretching increased bubble point, decreased the Gurley value
and increased the resistivity relative to untreated film (Example 17A).
While low resistivity is desirable, Examples 17C-17F demonstrate that gas
flow through the film can be enhanced without unduly increasing
resistivity. Example 17C showed that even a small amount of stretching of
a film that had been previously heated at 130.degree. C. (Example 17B)
provided a significant increase in bubble point and a significant decrease
in Gurley value while not significantly increasing the resistivity.
Examples 17D-17F illustrate the effects of a single step process of
stretching the film from Example 17A at a higher temperature than the
melting point of the polymer. Stretching at higher temperature resulted in
an even larger increase in bubble point and even larger decrease in Gurley
value. The change in resistivity varied with the amount of stretching from
almost no change (Example 17D) to a moderate change (Example 17E), and to
a somewhat larger change (Example 17F). Advancing and receding contact
angles (water) for the film were, respectively, (17D)
148.degree..+-.6.degree. and 95.degree..+-.5.degree., (17E)
153.degree..+-.4.degree. and 98.degree..+-.5.degree., and (17F)
156.degree..+-.8.degree. and 104.degree..+-.4.degree.. The results are
unexpected, in that unfilled porous films heated at or near their melting
point generally would collapse and become a dense, nonporous films.
Polarization curves for Example 17 are given in FIG. 12.
As shown in Example 18 below, as little as 20 volume % carbon relative to
the volume of HDPE in conjunction with a high loading of metallic
particles is sufficient to hinder densification of the membrane upon
heating at 130.degree. C. The peak melting temperature of the HDPE was
126.degree. C. as determined by DSC. Example 18 also shows that useful
TIPT films can be prepared using conductive metal particles in conjunction
with conductive carbon particles.
Example 18
TIPT Films Loaded with Non-Carbon Conductive Particles
Example 18A: A dispersion was prepared by wetting out 11,574 g of tungsten
powder having a primary particle size of 0.5 .mu.m (Teledyne Wah Chang,
Huntsville, Ala.) in 2576 g of mineral oil (Superla.TM. White Mineral Oil
No. 31) and 359 g of OLOA 1200 using a dispersator having a 2 in. sawtooth
disc head (Premier Mill Corp.). The resultant mixture was then milled by
recirculating this mixture through a 0.25 L horizontal mill (Premier Mill
Corp.) that contained a 50 vol. % charge of 1.3 mm steel beads, for 2 hr.
The resultant dispersion was then filtered through a 20 micron rope-wound
filter that had been pre-wet with oil. An iterative series of density
checks followed by oil additions was performed to adjust the density until
the desired target density of 3.6358 was reached.
As described in Example 1, this dispersion was pumped at 59.6 ml/min. into
an intermediate zone of a 25 mm twin screw operated at 90 rpm and HDPE
(grade GM 9255 from Hoechst Celanese Corp., now available as grade 1285
from Fina Oil & Chemical Co.) was gravimetrically metered into the
extruder throat at 0.54 kg (1.2 lb)/hr. The film was cast onto a smooth
casting wheel maintained at 32.degree. C. at about 0.225 mm thick. The oil
was extracted using three--15 min. washes of Vertrel 423.TM. and dried in
an exhaust hood. The resultant film was evaluated after washing/drying and
then after heating for 10 min. at 130.degree. C., as shown in Table 8. The
calculated weight percent of tungsten in the dried film was 95.0.
Example 18B: A membrane similar to Example 18A was prepared that had the
same volume percent loading of particulate, 48.3 vol. %, except that the
total particulate contained 73 volume % tungsten and 27 volume %
conductive carbon. The total weight percent particulate in the final
membrane was 93.5% of a 96.29/3.71 by weight mixture of tungsten and
Conductex 975.TM. conductive carbon (Colombian Chemicals Co.).
The dispersion was prepared by combining 2400 g of mineral oil (0.863 g/cc)
and 300 g of OLOA 1200 (0.92 g/cc). Then, 8880 g of tungsten (19.35 g/cc)
was wetted out into this mixture using a dispersator equipped with a 2 in.
sawtooth disc head (Premier Mill Corp.). A 341 g quantity of Conductex 975
(2.0 g/cc) was added in portions. Heating was commenced to lower the
dispersion viscosity to facilitate wetting out of the carbon. The
dispersion was then heated to 150.degree. C. for 20 min. The hot
dispersion was recirculated for one hour through a 0.25 L horizontal mill
operated at 3500 rpm. The mill contained an 80 vol. % charge of 1.3 mm
steel beads. The dispersion density was adjusted by adding more mineral
oil until a final density of 2.8922 g/cc at 25.degree. C. was reached.
As described in Example 1, the dispersion was pumped at 59.6 ml/min. into
an intermediate zone of a 25 mm twin screw operated at 90 rpm and HDPE
(grade GM 9255 from Hoechst Celanese Corp., now available as grade 1285
from Fina Oil & Chemical Co.) was gravimetrically metered into the
extruder throat at 0.54 kg (1.2 lb)/hr. The film was cast onto a smooth
casting wheel maintained at 32.degree. C. at about 0.225 mm thick. The oil
was extracted using three--15 min. washes of Vertrel 423.TM. and dried in
an exhaust hood. The resultant film was evaluated after washing/drying and
then after heating for 10 min. at 130.degree. C., as shown in Table 8.
TABLE 8
Ex- Caliper, Bubble Gurley no., Resistivity,
ample Treatment mm Point, .mu.m sec./50 cc ohm-cm
18A(1) None 0.24 0.27 135 >10.sup.6
18A(2) 130.degree. C./10 0.163 0.071 * >10.sup.6
min.
18B(1) None 0.173 0.10 317 101
18B(2) 130.degree. C./10 0.133 0.18 152 3.5
min.
*Film broke in Gurley instrument
The data shown in Table 8 indicate that conductive particles other than
carbon can be used to prepare TIPT films useful in the invention if at
least a minor amount of conductive carbon is included in order to achieve
acceptably low resistivity, decreased Gurley and increased bubble point on
heating the film.
Examples 19-27
Carbon-Loaded Porous PTFE Membranes--PF Process
In examples 19-27, the carbon loaded Teflon.RTM. (PTFE) media was prepared
using the general process taught, e.g., in U.S. Pat. No. 5,071,610,
incorporated herein by reference. In brief, the porous, conducting
Teflon.RTM. based membranes were prepared by hand mixing carbon particles,
a liquid dispersant and PTFE powder to form a putty-like mass. The
material then was passed multiple times through a heated mill (Model 4037,
Reliable Rubber and Plastic Machinery Co. Inc., North Bergen, N.J.), with
repeated folding and rotating of the sample and reductions of the mill gap
in between passes through the mill. The final membrane sheet was then
heated above the boiling point of the dispersant, in a vented oven, to
remove the dispersant.
The dispersant used in all the examples was Fluorinert.TM., FC-40
(b.p.=155.degree. C.) highly fluorinated electronic liquid, available from
3M Co., St. Paul, Minn. The use of a fluorinated dispersant in the PF
process is described in U.S. Pat. No. 5,113,860, incorporated herein by
reference.
The Teflon.RTM. binder, provided in dry form, was PTFE type 6-C., (DuPont
Chemical Co., Wilmington, Del.). Carbon particles consisted of carbon
black material and/or carbon fibers. The carbon black material is
identified in each example.
Carbon fibers were obtained from Strem Chemicals Inc., Newburyport, Mass.,
catalog number 06-0140. The approximately 6 mm long.times.0.001 cm
diameter fibers were received bundled randomly together and had to be
physically dispersed prior to use. This was done by brushing the fiber
bundles with a brass bristle brush to cause separated fibers to fall into
a USA Standard Testing Sieve (W. S. Tyler Inc., Mentor, Ohio), then
shaking on a sieve (100 mesh) shaker (W. S. Tyler Inc., Mentor, Ohio) for
one hour. The individual carbon fibers then were blended with carbon black
and added to the Teflon.RTM. and Fluorinert mixture.
In the following examples, the Gurley, resistance, contact angles and fuel
cell performance of several carbon/PTFE composite membranes are compared
to the ELAT.TM. PTFE/carbon material described in previous examples.
Example 19
PTFE/Carbon Black (95%) Membrane
Five grams of carbon black(Vulcan XC72R, Cabot Corp., Waltham, Mass.,
average particle diameter of 30 nm) were mixed with 0.263 g of PTFE and 40
g of Fluorinert.TM. FC-40. The mixture was hand-kneaded and formed as
described above, into a porous, conducting membrane 0.38 mm thick. The
membrane was dried in a vented oven at 180.degree. C. for one hour. The
resultant membrane, measuring approximately 37.5 cm.times.30 cm, was
approximately 95% by weight carbon.
A Gurley value of 37 seconds per 10 cc was measured for the membrane (FIG.
9).
Example 20
Membrane Comprising PTFE and Carbon Black/Carbon Fiber (89/6) Mixture
A 4.7 gram portion of carbon black, type Vulcan XC72R, and 0.3 g of carbon
fibers (Strem Chemicals Inc.) were mixed with 0.263 g of PTFE and 40 g of
Fluorinert.TM. FC-40. The mixture was hand kneaded and formed into a
porous, conducting membrane 0.38 mm thick. The membrane was dried in a
vented oven at 160.degree. C. for one hour. It was then folded in half and
passed through the mill rolls to a thickness of 0.30 mm. The resultant
membrane, measuring approximately 37.5 cm.times.30 cm, was approximately
95% by weight total carbon; 89% carbon black and 6% carbon fibers.
The measured Gurley was 21.5 seconds per 10 cc (FIG. 9). The resistance of
two 5 cm.sup.2 pieces of the membrane compressed to 0.51 cm thick,
measured in the fuel cell test cell as described in Example 6, was 4.0
milliohms, compared to 5.7 milliohms for two similar sized pieces of
ELAT.TM. reference material. (FIG. 10). This corresponds to a bulk
resistivity of 0.94 ohm-cm.
Example 21
Membrane Comprising PTFE and Carbon Black/Carbon Particle Mixture
A 3.0 gram portion of Vulcan XC72R carbon black and 2.0 g of Norit SX1
carbon particles, average particle size of 32-75 .mu.m (American Norit Co.
Inc., Atlanta, Ga.) were mixed with 0.263 g of PTFE and 40 g of
Fluorinert.TM. FC-40. The mixture was hand kneaded and formed into a
porous, conducting membrane 0.36 mm thick, as described above. The
membrane was dried in a vented oven. The resultant membrane was
approximately 95% by weight total carbon; 57% carbon black and 38% carbon
particles.
The measured Gurley was 35 seconds per 10 cc (FIG. 9). The resistance of
two 5 cm.sup.2 pieces of the membrane compressed to 0.076 cm thick was 4.0
milliohms (FIG. 10). This corresponds to a bulk resistivity of 0.26
ohm-cm. The advancing and receding contact angles were measured to be
153.+-.4.degree. and 113.7.+-.1.6.degree., respectively.
For examples 22, 23, 26 and 27, the same catalyst coated ion conducting
membrane was used to obtain the fuel cell polarization curves for the
different electrode backing material samples. The testing was done by
opening the cell after the completion of one test, removing the electrode
backing layer, and replacing them with the next electrode backing layer.
In FIG. 11 the order in which the samples were tested by reference to the
particular example was: Example 22, Example 23, Example 26, Example 27
followed by the ELAT control. Since the full performance of the catalyzed
Nafion 115 ion conduction membrane was obtained with the last sample using
the ELAT control, interchanging the electrode backing layers did not
damage the catalyzed membrane. As seen from the resistance and Gurley
measurements for these examples in FIGS. 9 and 10, the significant
differences in fuel cell performance cannot be due to resistance or just
porosity. The current limited performance of the sample from Example 26,
due to oxygen limited diffusion through a cathode water flooding layer, is
most likely associated with the lower porosity (higher Gurley value) and
much lower receding contact angle (107.5.degree.) compared to the other
examples in the series. These examples demonstrate that the wetting
characteristics of the type of carbon particle used is very important
since it influences the receding contact angle.
Example 22
Membrane Comprising PTFE and Carbon Black/Carbon Fibers (87/8)
A 4.6 gram portion of Vulcan XC72R carbon black and 0.4 g of carbon fibers
(Strem Chemicals Inc.) was mixed with 0.263 g of PTFE and 40 g of
Fluorinert.TM. FC-40. The mixture was hand kneaded and formed into a
porous, conducting membrane 0.28 mm thick. The membrane was dried in a
vented oven at 165.degree. C. for two hours. The resultant membrane was
approximately 95% by weight total carbon, 87% carbon black and
approximately 8% carbon fibers.
The measured Gurley was 2.1 seconds per 10 cc (FIG. 9). The resistance of
two 5 cm.sup.2 pieces of the membrane compressed to 0.058 cm thick was 9.6
milliohms (FIG. 10). This corresponds to a bulk resistivity of 0.82
ohm-cm. The advancing and receding contact angles were respectively
measured to be 154.+-.7.degree. and 132.+-.4.degree..
The fuel cell performance of the electrode backing layers prepared in this
example was measured using a Nafion.TM. 115 membrane-based 3-layer MEA
with nanostructured electrodes, as described in Example 9. FIG. 11 shows
the performance of this and other 5-layer MEAs of the invention, as well
as that of electrode backing layers prepared from ELAT.TM. reference
material . The current density of membranes of this Example at 0.5 volts
is seen to be 0.7 A/cm.sup.2. To obtain the fuel cell polarization curves
in FIG. 11, the fuel cell was operated at a temperature of 80.degree. C.
with a hydrogen pressure of 207 Kpa, an oxygen pressure of 414 Kpa, and
flow rates of 1 standard liter per minute, and the anode/cathode
humidification temperatures were 115.degree. C. and 80.degree. C.,
respectively.
Example 23
Membrane Comprising PTFE and Carbon Black/Carbon Fiber (78/7)
A 4.6 gram portion of Shawinigan C-55 carbon black, and 0.4 g of carbon
fibers (Strem Chemicals Inc.) was mixed with 0.90 g of PTFE and 45 g of
Fluorinert.TM. FC-40. The mixture was hand kneaded and formed into a
porous, conducting membrane 0.41 mm thick. After drying in a vented oven,
the resultant membrane was approximately 85% by weight carbon; 78% carbon
black and approximately 7% carbon fibers.
The measured Gurley was 6.2 seconds per 10 cc (FIG. 9). The resistance of
two 5 cm2 pieces 0.058 cm thick of the membrane was 10.6 milliohms (FIG.
10). This corresponds to a bulk resistivity of 0.90 ohm-cm. The advancing
and receding contact angles were respectively measured to be
157.+-.5.degree. and 137.+-.9.degree..
The fuel cell performance of the electrode backing layers prepared in this
example was measured as described above. The current density at 0.5 volts
was 0.95 A/cm.sup.2.
Example 24
Membrane Comprising PIFE and Carbon Black (92%)
This membrane was prepared as described in Example 19, except the total
carbon loading was 92% by weight Vulcan XC72R. The membrane thickness was
0.25 mm.
The measured Gurley was 24 seconds per 10 cc (FIG. 9), and the membrane
resistance was 20.5 milliohms (FIG. 10). This corresponds to a bulk
resistivity of 1.92 ohm-cm. The advancing and receding contact angles were
respectively measured to be 156.+-.8.degree. and 96.+-.5.degree..
Example 25
Membrane Comprising PTFE and Carbon Black (95%)
This membrane was prepared with the same ingredients as in Example 19
except a different thickness membrane was formed. The resultant membrane
was approximately 95% by weight carbon black and 0.32 mm thick.
The measured Gurley was 73 seconds per 10 cc (FIG. 9), and the membrane
resistance was 5.0 milliohms (FIG. 10), giving a bulk resistivity of 0.39
ohm-cm.
Example 26
Membrane Comprising PTFE and Carbon Black (90%)
A 90 wt % carbon-containing membrane was prepared as described in Example
19 using KetJen-600J carbon black. The porous, conducting membrane was
0.28 mm thick.
The measured Gurley was 27 seconds per 10 cc (FIG. 9), and the resistance
of two 5 cm pieces of the membrane was 5.0 milliohms (FIG. 10), for a bulk
resistivity of 0.48 ohm-cm. The advancing and receding contact angles were
respectively measured to be 161.+-.8.5.degree. and 107.5.+-.5.degree..
The fuel cell performance of the electrode backing layers prepared in this
example was measured as described above and shown in FIG. 11. The current
density at 0.5 volts was 0.28 A/cm.sup.2.
Example 27
Membrane Comprising PTFE and Carbon Black (85%)
An 85 wt % carbon-containing membrane was prepared as described in Example
19 using Shawingian C-55 carbon black. The porous, conducting membrane was
0.39 mm thick.
The measured Gurley was 4.4 seconds per 10 cc (FIG. 9), and the resistance
of two 5 cm.sup.2 pieces of the membrane was 13.4 milliohms (FIG. 10), for
a bulk resistivity of 0.88 ohm-cm. The advancing and receding contact
angles were respectively measured to be 157.+-.7.degree. and
141.+-.12.degree..
The fuel cell performance of the electrode backing layers prepared in this
example was measured as described above and shown in FIG. 11.
Example 28
Effect of TIPT Film Asymmetry
The carbon-filled HDPE membrane described in example 16B (heat treated) was
evaluated in a fuel cell under the same conditions as described above with
respect to Examples 9-14 except that Nafion.TM. 115 was used for the ion
conduction membrane and the electrode backing layers were extracted with
Vertrel 423. In Example 28A the film was placed with the side of the film
that was against a smooth casting wheel during quenching facing away from
the catalyzed membrane. In Example 28B the same film was placed with the
casting wheel side of the film facing towards the catalyzed membrane. SEM
photomicrographs of the casting wheel and air sides of film from 16B are
shown in FIG. 15, with comparable films without heat treatment shown in
FIG. 14. The fuel cell results are presented in FIG. 13. The results show
significantly better performance for Example 28B with the casting wheel
side of the film placed against the catalyzed membrane. As evident from
the SEM results in FIG. 15, the better results are obtained with the film
layer next to the catalyzed membrane having smaller pores and a denser
surface layer. FIG. 16 shows SEM micrographs for UHMWPE films
corresponding to Example 14 with and without heat treatment.
The embodiments described above are intended to be representative and not
limiting. Additional embodiments of the invention are within the claims.
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